[builtins] Use mcr for dmb instruction on armv6

At present compiler-rt cross compiles for armv6 ( -march=armv6 ) but includes
dmb instructions which are only available in armv7+ this causes SIGILL on
clang+compiler-rt compiled components on rpi0w platforms.

Reviewed By: MaskRay

Differential Revision: https://reviews.llvm.org/D99282