Some internal GL functions are exported to our libGLESv1_CM library
and to properly export them, they must be C functions.

Bug: angleproject:5534
Change-Id: I37280312f73fd5e55166e4fa36659267d657a50b
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2628139Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>