MIPS/gdbserver: Correctly handle narrow big-endian register transfers

MIPS/gdbserver: Correctly handle narrow big-endian register transfers

Fix an issue with `gdbserver' on big-endian n64 MIPS targets, where
$dspctl is 32-bit while the `ptrace' transfer data type is 64-bit.

Such register data is held in the low order 32 bits of the 64-bit data
quantity exchanged with the buffer used by `fetch_register' and
`store_register', however `supply_register' and `collect_register'
access the same data as a 32-bit quantity.  Consequently the register is
presented and written as all-zeros held in the most-significant part of
the big-endian 64-bit data quantity represented in the buffer:

(gdb) info registers
                  zero               at               v0               v1
 R0   0000000000000000 0000000000000001 0000000000000001 0000000000000000
                    a0               a1               a2               a3
 R4   00000001200212b0 0000000000000000 0000000000000021 000000012001a260
                    a4               a5               a6               a7
 R8   000000012001a260 0000000000000004 800000010c60c000 fffffffffffffff8
                    t0               t1               t2               t3
 R12  0000000000000000 000000fff7edab68 0000000000000001 0000000000000000
                    s0               s1               s2               s3
 R16  000000fff7ee2068 0000000120008b80 0000000000000000 0000000000000000
                    s4               s5               s6               s7
 R20  000000000052e5c8 000000000052f008 0000000000000000 0000000000000000
                    t8               t9               k0               k1
 R24  0000000000000000 00000001200027c0 0000000000000000 0000000000000000
                    gp               sp               s8               ra
 R28  00000001200212b0 000000ffffffc850 000000ffffffc850 0000000120005ee8
                status               lo               hi         badvaddr
      0000000000109cf3 0000000000943efe 000000000000000e 000000012001a008
                 cause               pc
      0000000000800024 0000000120005ee8
                  fcsr              fir              hi1              lo1
              0e800000         00f30000 0000000000000000 0101010101010101
                   hi2              lo2              hi3              lo3
      0202020202020202 0303030303030303 0404040404040404 0505050505050505
                dspctl          restart
              00000000 0000000000000000
(gdb)

Correct this problem then by using the `mips_supply_register'
`mips_collect_register' accessors for 32-bit registers where the
`ptrace' data type is 64-bit.  These accessors already operate on 32-bit
data quantities held in 64-bit containers:

(gdb) info registers
                  zero               at               v0               v1
 R0   0000000000000000 0000000000000001 0000000000000001 0000000000000000
                    a0               a1               a2               a3
 R4   00000001200212b0 0000000000000000 0000000000000021 000000012001a260
                    a4               a5               a6               a7
 R8   000000012001a260 0000000000000004 800000010d82e900 fffffffffffffff8
                    t0               t1               t2               t3
 R12  0000000000000000 000000fff7edab68 0000000000000001 0000000000000000
                    s0               s1               s2               s3
 R16  000000fff7ee2068 0000000120008b80 0000000000000000 0000000000000000
                    s4               s5               s6               s7
 R20  000000000052e5c8 000000000052f008 0000000000000000 0000000000000000
                    t8               t9               k0               k1
 R24  0000000000000000 00000001200027c0 0000000000000000 0000000000000000
                    gp               sp               s8               ra
 R28  00000001200212b0 000000ffffffc850 000000ffffffc850 0000000120005ee8
                status               lo               hi         badvaddr
      0000000000109cf3 0000000000943efe 000000000000000e 000000012001a008
                 cause               pc
      0000000000800024 0000000120005ee8
                  fcsr              fir              hi1              lo1
              0e800000         00f30000 0000000000000000 0101010101010101
                   hi2              lo2              hi3              lo3
      0202020202020202 0303030303030303 0404040404040404 0505050505050505
                dspctl          restart
              55aa33cc 0000000000000000
(gdb)

	gdb/gdbserver/
	* linux-mips-low.c [HAVE_PTRACE_GETREGS] (mips_collect_register)
	(mips_supply_register): Move outside HAVE_PTRACE_GETREGS.
	(mips_collect_ptrace_register, mips_supply_ptrace_register): New
	functions.
	(the_low_target): Wire them.