[builtins] Fix CPU feature detection for FreeBSD on AArch64 (#76532)

 [builtins] Fix CPU feature detection for FreeBSD on AArch64

This is a follow-up to #75635 which broke the build for FreeBSD on
AArch64:

```
compiler-rt/lib/builtins/cpu_model/aarch64/lse_atomics/freebsd.inc:3:16: error: call to undeclared function 'elf_aux_info'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    3 |   int result = elf_aux_info(AT_HWCAP, &hwcap, sizeof hwcap);
      |                ^
```

Using `elf_aux_info()` requires including `<sys/auxv.h>` first. To
prevent redeclaration issues with `hwcap.inc` attempting to define
`HWCAP_xxx` macros before `<sys/auxv.h>` does so, include `<sys/auxv.h>`
before any of the `.inc` files on FreeBSD.