[libc] Add diff and perf targets for more math functions

Comparing the run time of math functions from LLVM libc
with the MSVCRT libc:
|function	|perf-LLVM libc		    |perf-MSVCRT
|ceilf		|2.36 mins (141491389600 ns)|47.10 sec (47100940100 ns)
|exp2f             |6.37 mins (358441794700 ns)|12.39 mins (719404388300 ns)
|expf		|6.35 mins (381204661800 ns)|6.17 mins (346150163200 ns)
|fabsf		|1.18 mins (78425546600 ns) |53.75 sec (53745301900 ns)
|floorf		|3.15 mins (164770963800 ns)|45.94 sec (45935988400 ns)
|logbf		|4.38 mins (262508058800 ns)|55.47 sec (55466377700 ns)
|nearbyintf	|3.20 mins (167972868000 ns)|9.13 mins (523822963600 ns)
|rintf		|3.20 mins (168001498700 ns)|22.35 mins (1341266448800 ns)
|roundf		|2.35 mins (141151500600 ns)|1.42 mins (85326429800 ns)
|truncf		|2.31 mins (114846424000 ns)|59.41 sec (59414309100 ns)

Evaluating the number of differing results in Windows:
|function	|diff
|ceilf          |8388606 differing results
|exp2f         |213303887 differing results
|expf           |193922 differing results
|fabsf          |8388606 differing results
|floorf         |8388606 differing results
|logbf          |0 differing results
|nearbyintf     |0 differing results
|rintf          |0 differing results
|roundf         |0 differing results
|truncf 	|0 differing results

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D107462