arm64: Use .arch armv8-4.a with GNU as for extensions to tlbi

[jhb]

Summary

GNU as does not support a tlb-rmi architecture extension to selectively
enable access to this part of ARMv8.4-A. Instead, it has to be enabled
by selecting a suitable -march.

[jhb]

Using .arch here is not entirely future proof, but does work. The Linux kernel doesn't use .arch_extension. It figures out the highest configured -march= value, and for binutils it passes that to GNU as via -Wa,-march=<mumble> but for clang it uses .arch <mumble> via a special wrapper macro. That approach though spans the entire kernel build for GNU as, whereas this approach only affects pmap.c so will hopefully be easier to update if needed in the future.

For some related reading: https://www.sourceware.org/bugzilla/show_bug.cgi?id=26339

[andrew]

Why not use the new SYS_ARG macro? The tlbi instructions are in the sys instruction space.