libc/minimal: Create "real" functions for putc and putchar

When gcc is building without -fno-builtin, it will optimize calls like
printf("\n") into a call to putchar('\n'), but it won't use a static inline
in that case, instead insisting on a real function.

To make this a bit easier, adopt the usual C library practice of making
putc and putchar macros instead of static inline functions. There's no loss
of typechecking as the parameters are directly passed to underlying
functions with the same parameter types.

Signed-off-by: Keith Packard <keithp@keithp.com>