Modify the symbol table generation script to accept a list
of supported extensions. This allows for the EXT and
OES versions of the geometry shader extension to be exposed.

Test: angle_deqp_khr_gles31_tests --deqp-case=*geometry_shader*
Bug: angleproject:3571
Change-Id: Ia7127a03dbd3fce78957f0505d3ce0c9bab6cb15
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2765011
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>