hurd: Add enough auxv support for AT_ENTRY for PIE binaries

hurd: Add enough auxv support for AT_ENTRY for PIE binaries

Add PIE support for hurd, by faking an AT_ENTRY auxv entry.  That value
is expected to be read by svr4_exec_displacement, which will propagate
the executable displacement.

gdb/ChangeLog:

	* gdb/gnu-nat.c: Include <elf.h> and <link.h>.
	(gnu_xfer_auxv): New function.
	(gnu_xfer_partial): Call gnu_xfer_auxv when `object' is
	TARGET_OBJECT_AUXV.