ARM-specific error during compilation

[gb]

During compilation (often if not always during compilation of some file in "ccl:level-0;ARM;"), our build system sometimes gets the following error (or something very, very much like it):

> Error: value 0 is not of the expected type LIST.
> While executing: MEMEQL, in process listener(1).

This is ultimately due to the fact that *FEATURES* is getting bound to '(0 . 0) in COMPILE-FILE, but it's not clear how/why that happens.

In the backtrace in:

​http://setf.clozure.com:8010/builders/linuxarm/builds/72/steps/shell_2/logs/stdio

the local variable VALUES has the value ((0 . 0)), and *FEATURES* is bound to the CAR of that value (via PROGV.) VALUES is effectively bound to the value returned by:

(list (append nil *features*))

I've seen this happen, but haven't yet been able to determine why. There are lots of suspects ...

[gb]

I had this happen again, after fixing several bugs in pc_luser_xp().

GC integrity checks complain that the (0 . 0) points to free heap space, which suggests a bug in the forwarder (possibly forward_xp()).

[gb]

(In [14495]) arm-gc.c: GC integrity checks check cstack on ARM. arm-spentry.s: in progvsave: since we use the last physical word in the stack-consed gvector in which special-bindings are saved, the element-count of that gvector must be odd.

Fixes ticket:758.