Firefox needs to run during a PGO build to generate useful data,
use xvfb-run to provide a virtual X11 framebuffer to allow it
to do so.
Add --enable-rust-simd, even though SIMD is not in upstream rust yet
the firefox tarball provides it's own simd cargo crate to enable SIMD
Add --enable-elf-hack, more information about this can be found on the mozilla
Add --enable-stylo, since we are matching offical tarball performance, let's enable this.
Remove the flags workaround for GCC6, they are no longer required
Signed-off-by: Joey Riches <firstname.lastname@example.org>