[libc] Exclude Fuchsia from float128 detection (#73985)

Following from https://github.com/llvm/llvm-project/pull/73372:

Fuchsia targets currently don't support `float128`. Add detection for
`LIBC_TARGET_OS_IS_FUCHSIA`, and exclude this OS from setting
`LIBC_COMPILER_HAS_FLOAT128_EXTENSION`.