sim: nltvals: pull target syscalls out into a dedicated source file

sim: nltvals: pull target syscalls out into a dedicated source file

Like we just did for pulling out the errno map, pull out the syscall
maps into a dedicated common file.  Most newlib ports are using the
same syscall map, but not all, which means we have to do a bit more
work to migrate.

This commit adds the maps and switches the ports using the common
default syscall table over to it.  Ports using unique syscall tables
are still using the old targ-map.c logic.

Switching common ports over is easy by checking NL_TARGET, but the
ppc code needs a bit more cleanup here hence its larger diff.