Fix __builtin_bitreverse32 on 32-bit PowerPC

Fix __builtin_bitreverse32 on 32-bit PowerPC

This is a backport from LLVM 11.  Before this fix, code using
__builtin_bitreverse32 was crashing SIGILL because clang-10 emitted a
64-bit rldicl/clrldi instruction.  The SIGILL only happened on 32-bit
cpus, not on the G5.  The code for LLVM 11 uses __builtin_bitreverse,
so clang-10 needs this fix to build clang-11.

https://github.com/llvm/llvm-project/commit/a5d161c119d5a
https://reviews.llvm.org/D77946

ok kettenis@