[ARM][PR target/78945] Fix libatomic on armv7-m

ARM libatomic inline asm uses sel, uadd8, uadd16 instructions
which are only available if __ARM_FEATURE_SIMD32 is defined.

libatomic/
2017-01-30  Szabolcs Nagy  <szabolcs.nagy@arm.com>

	PR target/78945
	* config/arm/exch_n.c (libat_exchange): Check __ARM_FEATURE_SIMD32.

From-SVN: r245023