Add support for GCC Compilers, so it can be build on Ubuntu.

BUG=angleproject:3392

Change-Id: I2f5ac7355a40f52123bd465f86b1e966c59c943d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1592062Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Yi Xu <yiyix@chromium.org>