cris: Don't default to -mmul-bug-workaround

cris: Don't default to -mmul-bug-workaround

This flips the default for the errata handling for an old version
(TL;DR: workaround: no multiply instruction last on a cache-line).
Newer versions of the CRIS cpu don't have that bug.  While the impact
of the workaround is very marginal (coremark: less than .05% larger,
less than .0005% slower) it's an irritating pseudorandom factor when
assessing the impact of other changes.

Also, fix a wart requiring changes to more than TARGET_DEFAULT to flip
the default.

People building old kernels or operating systems to run on
ETRAX 100 LX are advised to pass "-mmul-bug-workaround".

gcc:
	* config/cris/cris.h (TARGET_DEFAULT): Don't include MASK_MUL_BUG.
	(MUL_BUG_ASM_DEFAULT): New macro.
	(MAYBE_AS_NO_MUL_BUG_ABORT): Define in terms of MUL_BUG_ASM_DEFAULT.
	* doc/invoke.texi (CRIS Options, -mmul-bug-workaround): Adjust
	accordingly.