s390x: support `.machine push` and `.machine pop` assembler directives

[folkertdev]

The documentation mentions that these directives should work (exactly like they do for powerpc):

.machine STRING[+EXTENSION]…
This directive allows changing the machine for which code is generated. string may be any of the -march= selection options, or push, or pop. .machine push saves the currently selected cpu, which may be restored with .machine pop.

But LLVM does not currently accept them in the ASM parser

This came up here rust-lang/rust#137720 (comment)

[uweigand]

@JonPsson1 @dominik-steenken

[uweigand]

Support added by #137302

[folkertdev]

Awesome, thanks!