build: strengthen 16 bit float support checks

build: strengthen 16 bit float support checks

Recent clang provides __bf16 on aarch64 but it is broken.

If built with -O0, the conversion is wrong:

    $ printf '\x3F\x80' | od --end=big -An -tfB | tr -d ' '
    1.875

If built with -O1 or higher, compilation fails:

    fatal error: error in backend:
    Cannot select: 0xb400007a58d29780: f32 = fp_extend 0xb40000...
    0xb40000...: bf16,ch = CopyFromReg 0xb40000..., Register:bf16 %13
    0xb40000...: bf16 = Register %13
    In function: print_bfloat

The latter issue does not cause the existing configure test to fail
because the promotion is optimized out.

* configure.ac: Ensure 16 bit float promotion code does not get
optimized out, and produces an expected result.