[mips] Support for +abs2008 attribute

[mips] Support for +abs2008 attribute

Instruction abs.[ds] is not generating correct result when working
with NaNs for revisions prior mips32r6 and mips64r6.

To generate a sequence which always produce a correct result, but also
to allow user more control on how his code is compiled, attribute
+abs2008 is added, so user can choose legacy or 2008.

By default legacy mode is used on revisions prior R6. Mips32r6 and
mips64r6 use abs2008 mode by default.

Differential Revision: https://reviews.llvm.org/D35983

llvm-svn: 352370