[libFuzzer][MSVC] Make Sanitizer Coverage MSVC-compatible

Summary:
Make Sanitizer Coverage work when compiled work when compiler-rt
is compiled with MSVC.

The previous solution did not work for MSVC because MSVC tried to
align the .SCOV$CZ section even though we used
__declspec(align(1)) on its only symbol:
__stop___sancov_cntrs.
Because the counter array is composed
of 1 byte elements, it does not always end on an 8 or 4 byte
boundary. This means that padding was sometimes added to
added to align the next section, .SCOV$CZ.
Use a different strategy now: instead of only instructing
the compiler not to align the symbol, make the section
one byte long by making its only symbol a uint8_t, so that
the linker won't try to align it.

Reviewers: morehouse, rnk

Reviewed By: rnk

Subscribers: kubamracek

Differential Revision: https://reviews.llvm.org/D56866

llvm-svn: 351714