kselftest: arm64: fake_sigreturn_bad_magic

Add a simple fake_sigreturn testcase which builds a ucontext_t with a bad
magic header and place it onto the stack. Expects a SIGSEGV on test PASS.

Introduce a common utility assembly trampoline function to invoke a
sigreturn while placing the provided sigframe at wanted alignment and
also an helper to make space when needed inside the sigframe reserved
area.

Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>