arm64/stacktrace: Export save_stack_trace_regs()

The ARM64 implements the save_stack_trace_regs function, but it is
unusable for any diagnostic tooling compiled as a kernel module due
the missing EXPORT_SYMBOL_GPL for the function.  Export
save_stack_trace_regs() to align with other architectures such as
s390, openrisc, and powerpc. This is similar to the ARM64 export of
save_stack_trace_tsk() added in git commit e27c7fa0.

Signed-off-by: William Cohen <wcohen@redhat.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>