s390/percpu: Fix MVIY_PERCPU() with older binutils

In the Linux kernel, the following vulnerability has been resolved:

s390/percpu: Fix MVIY_PERCPU() with older binutils

Commit a737737cdb9c ("s390/percpu: Infrastructure for more efficient
this_cpu operations") introduced MVIY_PERCPU(), which stringifies
arguments that are already C string literals. This generates an
assembler macro invocation with whitespace-separated quoted arguments:

GEN_MVIY "459712" "%r3"

GNU as versions prior to binutils 2.39 drop the separating whitespace
between quoted macro arguments during input scrubbing. They
consequently parse the invocation as a single argument and emit
repeated warnings:

Warning: missing closing `"'

The .ifc in GEN_MVIY never matches and GNU as exits successfully
without emitting the mviy instruction. As a result, the interrupted
per-CPU sequence is not marked in lowcore and the exception return
path cannot repair the per-CPU address register after migration.

All MVIY_PERCPU() callers pass C string literals. Use them directly
and separate the assembler macro arguments with an explicit comma. The
resulting invocation is:

GEN_MVIY 459712, %r3

This form is unambiguous for GNU as and LLVM's integrated assembler.
This behavior was fixed in GNU as from binutils 2.39, but Linux
supports binutils 2.30.