m68k: Use M68K_SCALE_AVAILABLE on __mpn_lshift and __mpn_rshift

This patch adds a new macro, M68K_SCALE_AVAILABLE, similar to gmp
scale_available_p (mpn/m68k/m68k-defs.m4) that expand to 1 if a
scale factor can be used in addressing modes.  This is used
instead of __mc68020__ for some optimization decisions.

Checked on a build for m68k-linux-gnu target mc68020 and mc68040.