Streamline Thumb config.

Enable Thumb2 iff ARM v7.  (We don't turn it on today for ARM <v7, and ARM v8 doesn't support it.)

BUG=skia:

Review URL: https://codereview.chromium.org/1054993002
diff --git a/gyp/common_conditions.gypi b/gyp/common_conditions.gypi
index 7f0b602..5f99eae 100644
--- a/gyp/common_conditions.gypi
+++ b/gyp/common_conditions.gypi
@@ -265,28 +265,10 @@
               '-fno-omit-frame-pointer',
             ],
           }],
-          [ 'skia_arch_type == "arm" and arm_thumb == 1', {
-            'cflags': [
-              '-mthumb',
-            ],
-            # The --fix-cortex-a8 switch enables a link-time workaround for
-            # an erratum in certain Cortex-A8 processors.  The workaround is
-            # enabled by default if you target the ARM v7-A arch profile.
-            # It can be enabled otherwise by specifying --fix-cortex-a8, or
-            # disabled unconditionally by specifying --no-fix-cortex-a8.
-            #
-            # The erratum only affects Thumb-2 code.
-            'conditions': [
-              [ 'arm_version < 7', {
-                'ldflags': [
-                  '-Wl,--fix-cortex-a8',
-                ],
-              }],
-            ],
-          }],
           [ 'skia_arch_type == "arm" and arm_version >= 7', {
             'cflags': [
               '-march=armv7-a',
+              '-mthumb',
             ],
             'ldflags': [
               '-march=armv7-a',