[mips] Mark fmaxl as a long double emulation routine

Failure was discovered upon running
projects/compiler-rt/test/builtins/Unit/divtc3_test.c
in a stage2 compiler build.

When compiling projects/compiler-rt/lib/builtins/divtc3.c,
a call to fmaxl within the divtc3 implementation had its
return values read from registers $2 and $3 instead of $f0 and $f2.
Include fmaxl in the list of long double emulation routines
to have its return value correctly interpreted as f128.

Almost exact issue here: https://reviews.llvm.org/D17760

Differential Revision: https://reviews.llvm.org/D52649

llvm-svn: 344326