aarch64: New Straight Line Speculation (SLS) mitigation flags

aarch64: New Straight Line Speculation (SLS) mitigation flags

Here we introduce the flags that will be used for straight line speculation.

The new flag introduced is `-mharden-sls=`.
This flag can take arguments of `none`, `all`, or a comma seperated list of one
or more of `retbr` or `blr`.
`none` indicates no special mitigation of the straight line speculation
vulnerability.
`all` requests all mitigations currently implemented.
`retbr` requests that the RET and BR instructions have a speculation barrier
inserted after them.
`blr` requests that BLR instructions are replaced by a BL to a function stub
using a BR with a speculation barrier after it.

Setting this on a per-function basis using attributes or the like is not
enabled, but may be in the future.

gcc/ChangeLog:

2020-06-02  Matthew Malcomson  <matthew.malcomson@arm.com>

	* config/aarch64/aarch64-protos.h (aarch64_harden_sls_retbr_p):
	New.
	(aarch64_harden_sls_blr_p): New.
	* config/aarch64/aarch64.c (enum aarch64_sls_hardening_type):
	New.
	(aarch64_harden_sls_retbr_p): New.
	(aarch64_harden_sls_blr_p): New.
	(aarch64_validate_sls_mitigation): New.
	(aarch64_override_options): Parse options for SLS mitigation.
	* config/aarch64/aarch64.opt (-mharden-sls): New option.
	* doc/invoke.texi: Document new option.