feat(security): add support for SLS mitigation

This patch enables support for the gcc compiler option "-mharden-sls",
the default is not to use this option. Setting HARDEN_SLS=1 sets
"-mharden-sls=all" that enables all hardening against straight line
speculation.

Signed-off-by: Bipin Ravi <bipin.ravi@arm.com>
Change-Id: I59f5963c22431571f5aebe7e0c5642b32362f4c9
3 files changed