[compiler-rt] Do not use ldrexd or strexd on v7M

The ldrexd and strexd instructions are undefined for the ARMv7M
architecture, so we cannot use them to implement the
__sync_fetch_and_*_8 builtins. There is no other way to implement
these without OS support, so this patch #ifdef's these functions out
for M-class architectures.

There are no tests as I cannot find any existing tests for these
builtins.

I used the __ARM_ARCH_PROFILE predefine because __ARM_FEATURE_LDREX is
deprecated and not set by clang.