Add support for GCC Compilers 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/+/1592062 Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Yi Xu <yiyix@chromium.org>