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'
1 file changed