newlib: export abs_newlib_basedir for all subdirs

newlib: export abs_newlib_basedir for all subdirs

When using the top-level configure script but subdir Makefiles, the
newlib_basedir value gets a bit out of sync: it's relative to where
configure lives, not where the Makefile lives.  Move the abs setting
from the top-level configure script into acinclude.m4 so we can rely
on it being available everywhere.  Although this commit doesn't use
it anywhere, just lays the groundwork.