Fix warnings in MIPS build

Fix warnings in MIPS build

This patch fixes the rest of the set-but-not-used warnings in the MIPS
code by using attribute_unused and also fixes some conversion warnings
by changing __arch_compare_and_exchange_xxx_[8|16]_int.  Assigning __cmp
(int type) to __prev (pointer type) generates a warning.  Assigning zero
to each separately avoids this warning without having to add a cast.

Signed-off-by: Steve Ellcey <sellcey@mips.com>
Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>