[scudo] Refactor of CRC32 and ARM runtime CRC32 detection

Summary:
ARM & AArch64 runtime detection for hardware support of CRC32 has been added
via check of the AT_HWVAL auxiliary vector.

Following Michal's suggestions in D28417, the CRC32 code has been further
changed and looks better now. When compiled with full relro (which is strongly
suggested to benefit from additional hardening), the weak symbol for
computeHardwareCRC32 is read-only and the assembly generated is fairly clean
and straight forward. As suggested, an additional optimization is to skip
the runtime check if SSE 4.2 has been enabled globally, as opposed to only
for scudo_crc32.cpp.

scudo_crc32.h has no purpose anymore and was removed.

Reviewers: alekseyshl, kcc, rengolin, mgorny, phosek

Reviewed By: rengolin, mgorny

Subscribers: aemerson, rengolin, llvm-commits

Differential Revision: https://reviews.llvm.org/D28574

llvm-svn: 292409