[libc] Workaround for GCC on `typedef` for `_Complex __float128` (#157010)

Currently, GCC can't parse `typedef _Complex __float128 cfloat128;`,
although `__typeof__` can be used as a workaround. Reported
https://gcc.gnu.org/PR121799 which was later considered as duplicate of
https://gcc.gnu.org/PR32187.

Some recent changes exposed it to GCC and then caused CI failure for
libc++. This patch adds a workaround for GCC.

GitOrigin-RevId: 33a583d37795285553fde30c75a4fec3243e8bd0