[libc] Switch functions to using global headers
This switches all of the files in src/string, src/math, and
test/src/math from using relative paths (e.g. #include “include/string.h”)
to global paths (e.g. #include <string.h>) to make bringing up those
functions on other platforms, such as fuchsia, easier.
Reviewed By: sivachandra
Differential Revision: https://reviews.llvm.org/D91394