libc: implement `getc()`

[ycsin]

This was originally added as a ticket under POSIX, but getc() is not a POSIX function. It is from C89.

https://en.cppreference.com/w/c/io/fgetc

The expectation from the POSIX API is that this function is implemented as part of whatever C library is in use.

[cfriedt]

Is this not already in the minimal libc??