libstdc++: Make _GLIBCXX_HOSTED respect -ffreestanding [PR103626]
This allows the library to switch to freestanding mode when compiling
with the -ffreestanding flag. This means you don't need a separate
libstdc++ build configured with --disable-hosted-libstdcxx in order to
compile for a freestanding environment.
The testsuite support files cannot be compiled for freestanding, so add
-fno-freestanding to override any -ffreestanding in the test flags.