[scudo] Separate hardware CRC32 routines

Summary:
As raised in D28304, enabling SSE 4.2 for the whole Scudo tree leads to the
emission of SSE 4.2 instructions everywhere, while the runtime checks only
applied to the CRC32 computing function.

This patch separates the CRC32 function taking advantage of the hardware into
its own file, and only enabled -msse4.2 for that file, if detected to be
supported by the compiler.

Another consequence of removing SSE4.2 globally is realizing that memcpy were
not being optimized, which turned out to be due to the -fno-builtin in
SANITIZER_COMMON_CFLAGS. So we now explicitely enable builtins for Scudo.

The resulting assembly looks good, with some CALLs are introduced instead of
the CRC32 code being inlined.

Reviewers: kcc, mgorny, alekseyshl

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D28417

llvm-svn: 291570