Fix MSVC cross compilation by Faless · Pull Request #7079 · libgit2/libgit2

Currently, the DefaultCFlags.cmake overrides the
CMAKE_STATIC_LINKER_FLAGS to suppress linker warnings about files with
no symbols defined.

This has the side effect of breaking MSVC cross compilation (where
CMAKE_STATIC_LINKER_FLAGS is used to specify the /MACHINE:ARCH flag)

This commit make sure we append to CMAKE_STATIC_LINKER_FLAGS instead of
replacing its values