[CMake][WinMsvc] Fix user passed compiler/linker flags

Users could pass flags by environment variables like CFLAGS/CXXFLAGS/LDFLAGS
or by using CMAKE_<LANG>_FLAGS_INIT/CMAKE_<t>_LINKER_FLAGS_INIT. So this
toolchain file should append to INIT flags instead. Otherwise, user
flags would be discarded here by assigning to CMAKE_<LANG>_FLAGS
directly.

Reviewed By: smeenai, mstorsjo

Differential Revision: https://reviews.llvm.org/D116709