Clang/MIPS: Use -mnan value for -mabs if not specified (#71157)

On most hardware, FCSR.ABS2008 is set the value same with FCSR.NAN2008.
Let's use this behaivor by default.

With this commit, `clang -target mips -mnan=2008 -c fabs.c` will imply
`-mabs=2008`.

And of course, `clang -mnan=2008 -mabs=legacy` can continue workable
like previous.

Co-authored-by: YunQiang Su <yunqiang.su@cipunited.com>