Libco: `asm` instead of `__asm__` breaks libretro-uae building on Android aarch64

This breaks libretro-uae (Amiga PUAE core) compilation if I update libco to latest version in that core. It compiles fine if I change `asm` to `__asm__` which is what it used to be:

https://github.com/libretro/libretro-common/blob/f88c23f5c64bed490b3e61d582f4c20cf3925ed1/libco/aarch64.c#L25