jump: Also handle jumps wrapped in UNSPEC or UNSPEC_VOLATILE

jump: Also handle jumps wrapped in UNSPEC or UNSPEC_VOLATILE

VAX has interlocked branch instructions used for atomic operations and
we want to have them wrapped in UNSPEC_VOLATILE so as not to have code
carried across.  This however breaks with jump optimization and leads
to an ICE in the build of libbacktrace like:

.../libbacktrace/mmap.c:190:1: internal compiler error: in fixup_reorder_chain, at cfgrtl.c:3934
  190 | }
      | ^
0x1087d46b fixup_reorder_chain
	.../gcc/cfgrtl.c:3934
0x1087f29f cfg_layout_finalize()
	.../gcc/cfgrtl.c:4447
0x1087c74f execute
	.../gcc/cfgrtl.c:3662

on RTL like:

(jump_insn 18 17 150 4 (unspec_volatile [
            (set (pc)
                (if_then_else (eq (zero_extract:SI (mem/v:SI (reg/f:SI 23 [ _2 ]) [-1  S4 A32])
                            (const_int 1 [0x1])
                            (const_int 0 [0]))
                        (const_int 1 [0x1]))
                    (label_ref 20)
                    (pc)))
            (set (zero_extract:SI (mem/v:SI (reg/f:SI 23 [ _2 ]) [-1  S4 A32])
                    (const_int 1 [0x1])
                    (const_int 0 [0]))
                (const_int 1 [0x1]))
        ] 101) ".../libbacktrace/mmap.c":135:14 158 {jbbssisi}
     (nil)
 -> 20)

when those branches are enabled with a follow-up change.  Also showing
with:

FAIL: gcc.dg/pr61756.c (internal compiler error)

Handle branches wrapped in UNSPEC_VOLATILE then and, for consistency,
also in UNSPEC.  The presence of UNSPEC_VOLATILE will prevent such
branches from being removed as they won't be accepted by `onlyjump_p',
we just need to let them through.

	gcc/
	* jump.c (pc_set): Also accept a jump wrapped in UNSPEC or
	UNSPEC_VOLATILE.
	(any_uncondjump_p, any_condjump_p): Update comment accordingly.