Implement strtod in native C code as part of libc.

This is a follow-up on Pull Request #891. `strtod` and all related functions are moved to libc side. The internal emscripten version is also bumped.

IMHO this is only a temp fix. When we are introducing a full libc impl, such as musl, newlib, etc. This file can be removed.

Three related test cases `test_strtod`, `test_sscanf` and `test_stdlibs` are all tested and passed. `test_strtod` is also changed to include the new case.

Thanks for all the help!