[cmake] Use -ffunction-sections and -Wl,--gc-sections on MinGW targets

If compiling with GCC or linking with ld.bfd, these options have little
effect, but if built with Clang and linked with LLD, they provide a
quite notable size decrease - this shrinks an entire llvm-mingw
distribution package by 22%.

If building with BUILD_SHARED_LIBS or LLVM_BUILD_LLVM_DYLIB with LLD,
this requires a version of LLD that contains a fix for auto exporting
symbols from comdats, 2b01a417.

Differential Revision: https://reviews.llvm.org/D101568