[libc] cleanup changes to gettimeofday.

+ Deleted duplicate definitions of StructTimeVal and StructTimeValPtr.
+ Caled syscall  clock_gettime to get timespec data.
+ Added tests to test for sleeping 200 and 1000 microseconds.
+ Fixed comments from https://reviews.llvm.org/D137881

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D138064