[PATCH] m68k: fix clobbering a5 in setjmp() [BZ #24202]

[Nick Alcock]

On 10 Feb 2019, Sergei Trofimovich uttered the following:

> setjmp() uses C code to store current registers into jmp_buf
> environment. -fstack-protector-all places canary into setjmp()
> prologue and clobbers 'a5' before it gets saved.
>
> The change inhibits stack canary injection to avoid clobber.

LGTM. I tried to identify everything that depended on clobbering no regs
and inhibit stack-protection in all of them, but clearly failed to spot
several setjmp implementations. Sorry!