Silence a warning from the test harness for unthreaded builds.

"The variable THREADS is defined but never used.", so, in WITH-TEST only
bind THREADS under #+SB-THREAD as all places where this variable is used
are guarded by this feature expression, too.
Improve the failure output of the test :range-reduction :x87.

Namely, output the function, argument, expected and actual result.
Prevent a make-array transform from modifying source forms.

That causes repeated compilations of inlined functions to accumulate
changes made by the transform.

Thanks to Bart Botta.
(regression since 1.0.42.11-bis)
Fix a test-case in threads.impure.lisp on -sb-thread.

A test was using make-thread without ensuring that sb-thread is enabled.

Fixes lp#1141795
New keyword argument for SB-C:DEFKNOWN: :overwrite-fndb-silently

 While not useful in the compiler itself, some libraries (not only
 contribs) define their own VOPs, transforms, etc., and the error
 when clobbering pre-existing defknowns made these libraries hard
 to reload.

 Use with :allow-other-keys for backward compatibility.