[libc] Disable float16 on 32-bit x86 without SSE2 (#219675)

Fixes #219668

Building llvm 23.1.0 (and current main) for 32-bit x86 without SSE2
fails since APFloat.cpp started including libc's shared/math.h. All the
errors come from the float16 headers:

```
libc/src/__support/FPUtil/BasicOperations.h:63:67: error: SSE register return with SSE2 disabled
libc/src/__support/math/acosf16.h:73:14: error: invalid conversion from type '_Float16' without option '-msse2'
```

The float16 detection in float16-macros.h checks __FLT16_MANT_DIG__.
Since GCC 14 that macro is defined on ia32 even without SSE2, where
_Float16 is storage-only and any arithmetic or returning by value is an
error.
The GCC 14 release notes say to check __SSE2__ for arithmetic support
instead: https://gcc.gnu.org/gcc-14/changes.html

So require SSE2 on 32-bit x86 in the guard, same as the existing arm32
and riscv exclusions in this file. x86-64 is not affected (__i386__ is
not defined there), and i386 with -msse2 keeps float16.

Tested with gcc 16.2 targeting i686: the reproducer from the issue goes
from ~340 errors to compiling clean, and a full 32-bit multilib build of
llvm 23.1.0 in Yocto succeeds with this change. Since 23.1.0 is
affected, I would like to request a backport to release/23.x once this
lands.

GitOrigin-RevId: 05da143c63d333420b99eab198e1a89c97480929