[libcxx] Consistently set CMAKE_STATIC_LIBRARY_PREFIX regardless of LIBCXX_ENABLE_STATIC

CMAKE_STATIC_LIBRARY_PREFIX affects the naming of all static libs (in
MSVC configurations), including c++experimental, which only is produced
as static regardless of LIBCXX_ENABLE_STATIC.

Differential Revision: https://reviews.llvm.org/D99176

GitOrigin-RevId: 7a804c09798a7051bd9189a0e59d84ea30b1e337
1 file changed