[libc] tweak stdio routines to be a bit more standard

change getc/putc/getchar/putchar style routines to take a fake
FILE pointer.
Move log2 to log2_uint to not collide with stdc log2()
[libc] fixup users of newly modified stdio routines
[app][tests] disable printf formatting warnings for printf test
[make] remove -fno-builtin

fno-builtin was mostly unnecessary before, but required some libc
work to run without it, due to standard function signature mismatches.