[MC] Add support for generating COFF CRCs

[MC] Add support for generating COFF CRCs

COFF sections are accompanied with an auxiliary symbol which includes a
checksum.  This checksum used to be filled with just zero but this seems
to upset LINK.exe when it is processing a /INCREMENTAL link job.
Instead, fill the CheckSum field with the JamCRC of the section
contents.  This matches MSVC's behavior.

This fixes PR19666.

N.B.  A rather simple implementation of JamCRC is given.  It implements
a byte-wise calculation using the method given by Sarwate.  There are
implementations with higher throughput like slice-by-eight and making
use of PCLMULQDQ.  We can switch to one of those techniques if it turns
out to be a significant use of time.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@246590 91177308-0d34-0410-b5e6-96231b3b80d8