x86: Do not use __builtin_isinf_sign for _Float64x/long double

x86: Do not use __builtin_isinf_sign for _Float64x/long double

Neither gcc [1] nor clang [2] handles pseudo-normal numbers correctly
with the __builtin_isinf_sign, so disable its usage for _Float64x and
long double types.

This only affects x86, so add a new define __FP_BUILTIN_ISINF_SIGN_DENORMAL
to gate long double and related types to the libc function instead.

It fixes the regression on test-ldouble-isinf when built with clang:

Failure: isinf (pseudo_zero): Exception "Invalid operation" set
Failure: isinf (pseudo_inf): Exception "Invalid operation" set
Failure: isinf (pseudo_qnan): Exception "Invalid operation" set
Failure: isinf (pseudo_snan): Exception "Invalid operation" set
Failure: isinf (pseudo_unnormal): Exception "Invalid operation" set
Failure: isinf_downward (pseudo_zero): Exception "Invalid operation" set
Failure: isinf_downward (pseudo_inf): Exception "Invalid operation" set
Failure: isinf_downward (pseudo_qnan): Exception "Invalid operation" set
Failure: isinf_downward (pseudo_snan): Exception "Invalid operation" set
Failure: isinf_downward (pseudo_unnormal): Exception "Invalid operation" set
Failure: isinf_towardzero (pseudo_zero): Exception "Invalid operation" set
Failure: isinf_towardzero (pseudo_inf): Exception "Invalid operation" set
Failure: isinf_towardzero (pseudo_qnan): Exception "Invalid operation" set
Failure: isinf_towardzero (pseudo_snan): Exception "Invalid operation" set
Failure: isinf_towardzero (pseudo_unnormal): Exception "Invalid operation" set
Failure: isinf_upward (pseudo_zero): Exception "Invalid operation" set
Failure: isinf_upward (pseudo_inf): Exception "Invalid operation" set
Failure: isinf_upward (pseudo_qnan): Exception "Invalid operation" set
Failure: isinf_upward (pseudo_snan): Exception "Invalid operation" set
Failure: isinf_upward (pseudo_unnormal): Exception "Invalid operation" set

Checked on x86_64-linux-gnu with gcc-15 and clang-18.

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123173
[2] https://github.com/llvm/llvm-project/issues/172651

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>