[compiler-rt] Stop using x86 builtin on AArch64 with GCC (#93890)

Previously, building `multc3.c` on A64 with GCC 7 or up but 9 and lower
will attempt to reference `__builtin_copysignq`, an [x86-specific
intrinsic][1]:

```
$ gcc -c multc3.c
In file included from fp_lib.h:24,
                 from multc3.c:14:
multc3.c: In function '__multc3':
int_math.h:71:32: warning: implicit declaration of function '__builtin_copysignq'; did you mean '__builtin_copysign'? [-Wimplicit-function-declaration]
 #define crt_copysignf128(x, y) __builtin_copysignq((x), (y))
                                ^~~~~~~~~~~~~~~~~~~
```

This is because `__has_builtin` is from GCC 10, and defined to 0 at the
top of int_math.h for affected GCC versions, so the fallback definition
is used. But `__builtin_copysignq` is unavailable on A64.

Use version detection to find `__builtin_copysignf128` instead. It's
available since GCC 7 and [available][2] on both x86 and A64, given this
macro is only used when `CRT_HAS_IEEE_TF`.

---

I realize this is fixing a problem for an out-of-tree build
configuration, but help would be greatly appreciated. Rust
[builds](https://github.com/rust-lang/compiler-builtins) `multc3.c` with
GCC 8 and this mis-selection is causing [build
issues](https://github.com/rust-lang/rust/issues/125619) way downstream.

ref: d2ce3e9621411f3391def327f89e3a650918989f

[1]: https://gcc.gnu.org/onlinedocs/gcc/x86-Built-in-Functions.html
[2]: https://gcc.gnu.org/gcc-7/changes.html

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