This copies over pp_printf from wrpc-sw, builds it, and changes
trace.h to explicitly call pp_printf. Actually, it also
define mprintf to pp_printf.

The excplicit call is needed, or the --gc-functions will discard the
pp_printf code if no object file refers to it.  Later,
arch/lm32/ram.ld redirects mprintf to pp_printf, but that would be too
late.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>