[compiler-rt] [emutls] Handle unused parameters in a compiler agnostic way

The MSVC specific pragmas disable this warning, but the pragmas themselves
(when not guarded by any _MSC_VER ifdef) cause warnings for other targets,
e.g. when targeting mingw.

Instead silence the MSVC warnings about unused parameters by casting
the parameters to void.

Differential Revision: https://reviews.llvm.org/D91851