generate_gypi.sh: --disable-avx and --only-config flags

Add two flags to make the script more flexible:
--disable-avx: do not generate AVX and AVX2 sources.
--only-configs: only generate configuration header (i.e. no GN or GYP files).

TESTED=Ran:
./generate_gypi.sh
and verified nothing changed in the checkout.
./generate_gypi.sh --disable-avx
and verified AVX was disabled.
./generate_gypi.sh --only-configs
with a modified .gypi and verified it was not overwritten.

R=johannkoenig@google.com

Review URL: https://codereview.chromium.org/1339033002 .
1 file changed