x86_init_func_common: Add ENDBR at function entry

x86_init_func_common: Add ENDBR at function entry

Intel Control-flow Enforcement Technology (CET):

https://software.intel.com/en-us/articles/intel-sdm

when IBT is enabled, all indirect branch targets must start with ENDBR
instruction which is a NOP on non-CET processors.

Closes: https://gitlab.freedesktop.org/mesa/mesa/issues/2575
Acked-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Ben Widawsky <ben.widawsky@intel.com>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3985>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3985>