[libc++][print] Adds FILE functions.

Drive-by fix to make sure the __retarget_buffer works correctly whan
using a hint of 1. This was discovered in one of the new tests.

Drive-by fixes __retarget_buffer when initialized with size 1.

Implements parts of
- P2093R14 Formatted output
- P2539R4  Should the output of std::print to a terminal be
           synchronized with the underlying stream?

Reviewed By: #libc, ldionne

Differential Revision: https://reviews.llvm.org/D150044