x86: use raw opcode for xgetbv instruction

This allows the CPU detection to work with assemblers not supporting
the xgetbv mnemonic.  These include clang and some BSD versions.

All AVX code will be written for yasm, where the main assembler
is not involved.
Signed-off-by: Mans Rullgard <mans@mansr.com>

libavutil/x86/cpu.c

@@ -36,7 +36,7 @@
          : "0" (index));
 
 #define xgetbv(index,eax,edx)                                   \
-    __asm__ ("xgetbv" : "=a"(eax), "=d"(edx) : "c" (index))
+    __asm__ (".byte 0x0f, 0x01, 0xd0" : "=a"(eax), "=d"(edx) : "c" (index))
 
 /* Function to test if multimedia instructions are supported...  */
 int ff_get_cpu_flags_x86(void)