mips: Set dump-stack arch description

In the framework of the MIPS architecture the mips_set_machine_name()
method is defined to set the machine name. The name currently is only used
in the /proc/cpuinfo file content generation. Let's have it utilized to
mach-personalize the dump-stack data too in a way it's done on ARM, ARM64,
RISC-V, etc.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>