libc/minimal: Add -fno-builtin option

This ensures that the compiler won't end up recognizing patterns in code
that map to known built-in functions, like noticing that the memcpy
implementation looks a lot like a memory copy operation that could be
replaced by a call to ... memcpy.

When built with -ffreestanding, this has no effect as that option always
enables -fno-builtin.

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