[compiler-rt] Stop using x86 built-in functions on AArch64 with GCC

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

Usages of `__builtin_fabsf128` have the same issue.

Use version detection to find `__builtin_copysignf128` and
`__builtin_fabsf128`. They are available since GCC 7 and [available][2]
on both x86 and A64, given they are only used when `CRT_HAS_IEEE_TF`.

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

compiler-rt/lib/builtins/int_math.h

@@ -65,9 +65,12 @@
 #define crt_copysign(x, y) __builtin_copysign((x), (y))
 #define crt_copysignf(x, y) __builtin_copysignf((x), (y))
 #define crt_copysignl(x, y) __builtin_copysignl((x), (y))
-#if __has_builtin(__builtin_copysignf128)
+// We define __has_builtin to always return 0 for GCC versions below 10,
+// but __builtin_copysignf128 is available since version 7.
+#if __has_builtin(__builtin_copysignf128) ||                                   \
+    (defined(__GNUC__) && __GNUC__ >= 7)
 #define crt_copysignf128(x, y) __builtin_copysignf128((x), (y))
-#elif __has_builtin(__builtin_copysignq) || (defined(__GNUC__) && __GNUC__ >= 7)
+#elif __has_builtin(__builtin_copysignq)
 #define crt_copysignf128(x, y) __builtin_copysignq((x), (y))
 #endif
 #endif
@@ -80,9 +83,11 @@
 #define crt_fabs(x) __builtin_fabs((x))
 #define crt_fabsf(x) __builtin_fabsf((x))
 #define crt_fabsl(x) __builtin_fabsl((x))
-#if __has_builtin(__builtin_fabsf128)
+// We define __has_builtin to always return 0 for GCC versions below 10,
+// but __builtin_fabsf128 is available since version 7.
+#if __has_builtin(__builtin_fabsf128) || (defined(__GNUC__) && __GNUC__ >= 7)
 #define crt_fabsf128(x) __builtin_fabsf128((x))
-#elif __has_builtin(__builtin_fabsq) || (defined(__GNUC__) && __GNUC__ >= 7)
+#elif __has_builtin(__builtin_fabsq)
 #define crt_fabsf128(x) __builtin_fabsq((x))
 #endif
 #endif