support for "r" prefixed variables in ARM inline assembly

[nickdesaulniers]

ARMv8 added support for register prefixes "w" (eg w0, w1) for lower 32b and "x" (eg x0, x1) for full 64b of the same register. GCC also supports the ARMv7 "r" prefixes, but clang complains for ARMv8 that this is ambiguous.

LLVM Bug: https://llvm.org/pr36862
Chromium Bug: https://bugs.chromium.org/p/chromium/issues/detail?id=824526
Example Android workaround: https://android-review.googlesource.com/c/kernel/common/+/645181

[nickdesaulniers]

temp patch sent upstream to linux by @mkaehlcke: https://patchwork.kernel.org/patch/10302339/

[nickdesaulniers]

patch to LLVM started: https://reviews.llvm.org/D44815

[mkaehlcke]

https://reviews.llvm.org/D44815 has landed.