[libc] add scanf parser and core utilities

This is the first piece of scanf. It's very similar in design to printf,
and so much of the code is copied from that. There were potential issues
with conflicting macros so I've also renamed the "ASSERT_FORMAT_EQ"
macro for printf to "ASSERT_PFORMAT_EQ".

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D136288