kselftest: arm64: add helper get_current_context

Introduce a new common utility function get_current_context() which can be
used to grab a ucontext without the help of libc, and also to detect if
such ucontext has been successfully used by placing it on the stack as a
fake sigframe.

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>