centralize VECTORCALL as SK_VECTORCALL

Gonna start using this, might as well define it once centrally.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2126253002

No public API changes.
TBR=reed@google.com

Review-Url: https://codereview.chromium.org/2126253002
5 files changed