Because they may be returned as stdcall from eglGetProc address, the entry
point functions must be stdcall.

Fixes issues on 32-bit builds.

BUG=angle:733

Change-Id: Ia20046a67fdc28c702837ec59b8299c6bb722006
Reviewed-on: https://chromium-review.googlesource.com/233471Reviewed-by: Jamie Madill <jmadill@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>