[mips] Introducing option -mabs=[legacy/2008]

In patch r205628 using abs.[ds] instruction is forced, as they should behave
in accordance with flags Has2008 and ABS2008. Unfortunately for revisions
prior mips32r6 and mips64r6, abs.[ds] is not generating correct result when
working with NaNs. To generate a sequence which always produce a correct
result but also to allow user more control on how his code is compiled,
option -mabs is added where user can choose legacy or 2008.
By default legacy mode is used on revisions prior R6. Mips32r6 and mips64r6
use abs2008 mode by default.

Patch by Aleksandar Beserminji

Differential Revision: https://reviews.llvm.org/D35982

llvm-svn: 311669