cgo: add support for printf

The C printf function is sometimes needed for C files included using
CGo. This commit makes sure they're available on all systems where CGo
is fully supported (that is, everywhere except on AVR).

For baremetal systems using picolibc, I've picked the integer-only
version of printf to save on flash size. We might want to consider
providing a way to pick the floating point version instead, if needed.

builder/picolibc.go

@@ -29,6 +29,7 @@ var libPicolibc = Library{
 			"-D_HAVE_ALIAS_ATTRIBUTE",
 			"-DTINY_STDIO",
 			"-DPOSIX_IO",
+			"-DFORMAT_DEFAULT_INTEGER", // use __i_vfprintf and __i_vfscanf by default
 			"-D_IEEE_LIBM",
 			"-D__OBSOLETE_MATH_FLOAT=1", // use old math code that doesn't expect a FPU
 			"-D__OBSOLETE_MATH_DOUBLE=0",