MIPS: Allow the auxv's elf_platform entry to be set.

The userspace runtime linker uses the elf_platform to find the libraries
optimized for the current CPU archecture variant.  First we need to allow it
to be set to something other than NULL.  Follow-on patches will set some
values for specific CPUs.

GLIBC already does the right thing.  The kernel just needs to supply good
data.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Patchwork: http://patchwork.linux-mips.org/patch/891/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>