cpu.c: Fix LLVM compile

Switch from `asm` to `__asm__` which is better supported.

Patch-from: Olivier Tristan <o.tristan@uvi.net>

src/libFLAC/cpu.c

@@ -81,7 +81,7 @@ cpu_xgetbv_x86(void)
 	return (uint32_t)_xgetbv(0);
 #elif defined __GNUC__
 	uint32_t lo, hi;
-	asm volatile (".byte 0x0f, 0x01, 0xd0" : "=a"(lo), "=d"(hi) : "c" (0));
+	__asm__ volatile (".byte 0x0f, 0x01, 0xd0" : "=a"(lo), "=d"(hi) : "c" (0));
 	return lo;
 #else
 	return 0;