MIPS: Create and use asm/arch_hweight.h

Some MIPS ISA processor varients can do hweight operations
efficiently.

Split arch_hweight.h into a seperate file, and implement the
operations with __builtin_popcount{,ll} if supported.

Signed-off-by: David Daney <ddaney@caviumnetworks.com>
To: linux-mips@linux-mips.org
Cc: David Daney <ddaney@caviumnetworks.com>
Patchwork: https://patchwork.linux-mips.org/patch/1430/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>