x86_simd_caps: check max cpuid before testing AVX2

structured extended feature flags require eax = 7; avoids incorrectly
detecting avx2 on some older processors that support avx.

from [1]:
INPUT EAX = 0: Returns CPUID’s Highest Value for Basic Processor
Information and the Vendor Identification String

[1] http://www.intel.com/content/www/us/en/processors/processor-identification-cpuid-instruction-note.html

Change-Id: I6b4735b5f7b7729a815e428fca767d1e5a10bcab

vpx_ports/x86.h

diff --git a/vpx_ports/x86.h b/vpx_ports/x86.h
index ae349fb..7d93710 100644
--- a/vpx_ports/x86.h
+++ b/vpx_ports/x86.h
@@ -152,7 +152,7 @@
 x86_simd_caps(void) {
   unsigned int flags = 0;
   unsigned int mask = ~0;
-  unsigned int reg_eax, reg_ebx, reg_ecx, reg_edx;
+  unsigned int max_cpuid_val, reg_eax, reg_ebx, reg_ecx, reg_edx;
   char *env;
   (void)reg_ebx;
 
@@ -168,9 +168,9 @@
     mask = strtol(env, NULL, 0);
 
   /* Ensure that the CPUID instruction supports extended features */
-  cpuid(0, 0, reg_eax, reg_ebx, reg_ecx, reg_edx);
+  cpuid(0, 0, max_cpuid_val, reg_ebx, reg_ecx, reg_edx);
 
-  if (reg_eax < 1)
+  if (max_cpuid_val < 1)
     return 0;
 
   /* Get the standard feature flags */
@@ -193,10 +193,12 @@
     if ((xgetbv() & 0x6) == 0x6) {
       flags |= HAS_AVX;
 
-      /* Get the leaf 7 feature flags. Needed to check for AVX2 support */
-      cpuid(7, 0, reg_eax, reg_ebx, reg_ecx, reg_edx);
+      if (max_cpuid_val >= 7) {
+        /* Get the leaf 7 feature flags. Needed to check for AVX2 support */
+        cpuid(7, 0, reg_eax, reg_ebx, reg_ecx, reg_edx);
 
-      if (reg_ebx & BIT(5)) flags |= HAS_AVX2;
+        if (reg_ebx & BIT(5)) flags |= HAS_AVX2;
+      }
     }
   }