kselftests/arm64: add BTI paciasp test

BTI-paciasp functionality test checks when accessing a guarded memory
region and if PSTATE.BTYPE has a value of 0b01, 0b10 or 0b11 then BTI
instruction that is compatible with the current value of PSTATE.BTYPE
will not generate a Branch Target exception and allow execution of
subsequent instructions within the memory region. The Branch Target
Identification instructions are NOP in a non-guarded page.

Co-Developed-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Vikas Kumar <vikas.kumar2@arm.com>