Build: GNUARM: Restrict support for PACBTI to AC6

Restrict the branch protection feature to Arm Compiler 6
because GNUARM lacks full support.

Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>
Change-Id: I684819d9e68f830a1291f8b47f9bbd21d456ce1f
2 files changed