[libc] use <sys/syscall.h> as its cleaner and license compliant

Author: bojle

libc/include/sys/syscall.h.def

@@ -11,6 +11,9 @@
 
 //TODO: Handle non-linux syscalls
 
+#if defined(__APPLE__)
+#include <sys/syscall.h>
+#else
 #include <asm/unistd.h>
 
 #ifdef __NR_FAST_atomic_update
@@ -2361,5 +2364,6 @@
 #define SYS_writev __NR_writev
 #endif
 
+#endif
 
 #endif // LLVM_LIBC_SYS_SYSCALL_H