linuxthread/arm: Unlock ldrex/strex varsion of testandset for __ARM_ARCH >= 7

linuxthread/arm: Unlock ldrex/strex varsion of testandset for __ARM_ARCH >= 7

Thomas has repored failure building ARM 32-bit systems for ARMv8 cores

  CC libpthread/linuxthreads/mutex.os
/tmp/ccn8SFKU.s: Assembler messages:
/tmp/ccn8SFKU.s:162: Error: swp{b} use is obsoleted for ARMv8 and later
/tmp/ccn8SFKU.s:186: Error: swp{b} use is obsoleted for ARMv8 and later
/tmp/ccn8SFKU.s:203: Error: swp{b} use is obsoleted for ARMv8 and later
/tmp/ccn8SFKU.s:224: Error: swp{b} use is obsoleted for ARMv8 and later
make[1]: *** [Makerules:369: libpthread/linuxthreads/mutex.os] Error 1

This is due to libpthread/linuxthreads/sysdeps/arm/pt-machine.h which
uses the swp instruction that is not allowed on ARMv8.

All __ARM_ARCH >= 7 support ldrex/strex instructions, so unlock
testandset() varaint for them.

Reported-by: Thomas Petazzoni <[email protected]>
Signed-off-by: Vladimir Murzin <[email protected]>