[ARMv8] ARMv8 builds are capable of ARMv7 instructions.

Correct a small bug which throws a compile-time assertion for ARMv8
builds since this change:
  https://codereview.chromium.org/2223433002

This does not add full support for ARMv8 instructions; it simply sets
the CAN_USE_ARMV7_INSTRUCTIONS define on ARMv8 builds.

BUG=

Review-Url: https://codereview.chromium.org/2293253004
Cr-Commit-Position: refs/heads/master@{#39087}

BUILD.gn

@@ -200,7 +200,7 @@ config("toolchain") {
 
   if (v8_current_cpu == "arm") {
     defines += [ "V8_TARGET_ARCH_ARM" ]
-    if (arm_version == 7) {
+    if (arm_version >= 7) {
       defines += [ "CAN_USE_ARMV7_INSTRUCTIONS" ]
     }
     if (arm_fpu == "vfpv3-d16") {