arm64: module-plts: factor out PLT generation code for ftrace

To allow the ftrace trampoline code to reuse the PLT entry routines,
factor it out and move it into asm/module.h.

Cc: <stable@vger.kernel.org>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>