Fix BUILD.gn for arch other then x86 and x64

opts_gypi should only be defined for x86 and x64 else we get unused
variable error

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2248323002

Review-Url: https://codereview.chromium.org/2248323002
1 file changed