Fix unsupported instruction use in char/short_fetch_and_add for ARMv6

* src/atomic_ops/sysdeps/gcc/arm.h (AO_ARM_HAVE_LDREXBH): Define new
macro for ARMv6K/Z+ (excluding ARMv6, ARMv6J, ARMv6T2).
* src/atomic_ops/sysdeps/gcc/arm.h (AO_char_store, AO_short_store,
AO_char_fetch_and_add, AO_short_fetch_and_add): Define (together with
the corresponding AO_HAVE_x macros) only if AO_ARM_HAVE_LDREXBH (since
otherwise LDREXB/STREXB, LDREXH/STREXH are not supported by the target
processor).
* src/atomic_ops/sysdeps/gcc/arm.h: Include char_atomic_store.h and
short_atomic_store.h only if AO_HAVE_char_store is not defined (by
gcc/arm.h itself).