build: unbreak x86 builds

SSE is not guaranteed to be present on x86 hosts, so i386..i686
compilers default to -mno-sse. That however makes the build fail
because vmaf unconditionally uses emmintrin functions which are
not available under no-sse. ppc64le builds however are just fine, so...

Change the macro guarding emmintrin to not match on x86-like
architectures, but to match on the enablement of SSE2. The macro
"__SSE2__" is the gcc name and is true whenever -msse2 is enabled
(explicitly or implicitly); clang should behave the same. I have not
tested this under other compilers such as Microsoft, though.

Fixes: #374

libvmaf/src/feature/adm_tools.c

@@ -35,7 +35,7 @@
 #define MIN(x, y) (((x) < (y)) ? (x) : (y))
 #define MAX(x, y) (((x) > (y)) ? (x) : (y))
 
-#if ARCH_X86
+#ifdef __SSE2__
 #ifdef ADM_OPT_RECIP_DIVISION
 
 #include <emmintrin.h>
@@ -50,7 +50,7 @@ static float rcp_s(float x)
 #endif //ADM_OPT_RECIP_DIVISION
 #else
 #define DIVS(n, d) ((n) / (d))
-#endif //ARCH_X86
+#endif // __SSE2__
 
 static const float dwt2_db2_coeffs_lo_s[4] = { 0.482962913144690, 0.836516303737469, 0.224143868041857, -0.129409522550921 };
 static const float dwt2_db2_coeffs_hi_s[4] = { -0.129409522550921, -0.224143868041857, 0.836516303737469, -0.482962913144690 };