Add the fma() builtin function to ESSL.

Bug: angleproject:3569
Change-Id: Ic8419b4c117ecdd8b47aa733bf7aff9ee7579bbf
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1954484Reviewed-by: Jiajia Qin <jiajia.qin@intel.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>