GN: match shell behavior for cc and cxx in is_clang.py

Interprets cc/cxx as part of the shell command rather than literal
arguments. This matches the behavior of shell invocations from ninja.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2375573002

Review-Url: https://codereview.chromium.org/2375573002
1 file changed