Fix sb-gmp:mpz-pow for non-bignum bases

Don't declare types, and we don't need the check-type either
Fix for sb-gmp bignum result allocation (lp#1206191)

Patch based on 59e72c69c00abf9095d82e9a7b65f9c1b54d105c from
<https://github.com/sfrank/sb-gmp> (thanks to Stephan Frank)
Save some space in WITH-OUTPUT-TO-STRING.

Not passing :element-type 'character to MAKE-STRING-OUTPUT-STREAM
saves a few bytes, since it's the default value.
Remove a wrong IGNORE declaration.

An incorrect ignore declaration was used with #!-sb-package-locks,
even though the ignored variables are used without sb-package-locks
too.

Patch by Douglas Katzman.
Improve knownfun declarations.

Make some types more accurate, add some
:derive-type #'result-type-first/last-arg.
Add missing %adjoin/member/assoc-test/not/key defknowns.
Adjust SETcc instruction encoding on x86-64.

Avoid emitting REX for legacy registers.
LDB/DPB do not check for negative indexes.

Calling (lambda (x y) (ldb (byte x y) 100)) with -1 -2 didn't raise an
error.

Reported by Bart Botta.
sb-safepoint: Fix interrupts during GC.

In interrupt_handle_now, move

context_sap = alloc_sap(context);
info_sap = alloc_sap(info);

after WITH_GC_AT_SAFEPOINTS_ONLY(), otherwise the allocation will
break the running GC.
Fixes lp#1133018.
better comment describing INDEX type

merged ideas from Paul Khuong and Jan Moringen, from
<http://paste.lisp.org/display/138734>
better ctor fallback-generators

The logic surrounding default-initargs in the presence of "hairy"
methods on make-instance and friends was not quite right, leading
to evaluation of the wrong things at the wrong times.  Patch by
Jan Moringen with extra test cases (lp#1179858).