Fix 'undeclared identifier USRSTACK' compiler error on OpenBSD-6.2

Fix 'undeclared identifier USRSTACK' compiler error on OpenBSD-6.2

Issue #198 (bdwgc).

USRSTACK macro is defined in <machine/vmparam.h> but that is protected
by "ifdef _KERNEL" in <uvm/uvm_param.h>.

* include/private/gcconfig.h [POWERPC && OPENBSD && !GC_OPENBSD_THREADS]
(STACKBOTTOM): Add comment.
* include/private/gcconfig.h [(POWERPC || SPARC || I386 || MIPS
|| HP_PA || ALPHA || ARM32 || SH && !SH4 || X86_64) && OPENBSD
&& !GC_OPENBSD_THREADS] (STACKBOTTOM): Define only if USRSTACK is
defined.
* include/private/gcconfig.h [MIPS && OPENBSD]: Reformat code.