sbc_primitives: Fix build on non-x86.

Don't call __builtin_cpu_init unless targeting i386 or x86_64.
Otherwise we get an error at link time:

  CC       sbc/sbc_primitives.lo
sbc/sbc_primitives.c: In function ‘sbc_init_primitives_x86’:
sbc/sbc_primitives.c:596:2: warning: implicit declaration of function ‘__builtin_cpu_init’; did you mean ‘__builtin_irint’? [-Wimplicit-function-declaration]
[...]
  CCLD     src/sbcdec
ld: sbc/.libs/libsbc-private.a(sbc_primitives.o): in function `sbc_init_primitives':
sbc_primitives.c:(.text+0x3a30): undefined reference to `__builtin_cpu_init'
diff --git a/sbc/sbc_primitives.c b/sbc/sbc_primitives.c
index 97a75be..09c214a 100644
--- a/sbc/sbc_primitives.c
+++ b/sbc/sbc_primitives.c
@@ -593,6 +593,7 @@
 
 static void sbc_init_primitives_x86(struct sbc_encoder_state *state)
 {
+#if defined(__x86_64__) || defined(__i386__)
 	__builtin_cpu_init();
 
 #ifdef SBC_BUILD_WITH_MMX_SUPPORT
@@ -604,6 +605,7 @@
 	if (__builtin_cpu_supports("sse4.2"))
 		sbc_init_primitives_sse(state);
 #endif
+#endif
 }
 
 /*