libgcc/CET: Skip signal frames when unwinding shadow stack

When -fcf-protection -mcet is used, I got

FAIL: g++.dg/eh/sighandle.C

(gdb) bt
 #0  _Unwind_RaiseException (exc=exc@entry=0x416ed0)
    at /export/gnu/import/git/sources/gcc/libgcc/unwind.inc:140
 #1  0x00007ffff7d9936b in __cxxabiv1::__cxa_throw (obj=<optimized out>,
    tinfo=0x403dd0 <typeinfo for int@@CXXABI_1.3>, dest=0x0)
    at /export/gnu/import/git/sources/gcc/libstdc++-v3/libsupc++/eh_throw.cc:90
 #2  0x0000000000401255 in sighandler (signo=11, si=0x7fffffffd6f8,
    uc=0x7fffffffd5c0)
    at /export/gnu/import/git/sources/gcc/gcc/testsuite/g++.dg/eh/sighandle.C:9
 #3  <signal handler called> <<<< Signal frame which isn't on shadow stack
 #4  dosegv ()
    at /export/gnu/import/git/sources/gcc/gcc/testsuite/g++.dg/eh/sighandle.C:14
 #5  0x00000000004012e3 in main ()
    at /export/gnu/import/git/sources/gcc/gcc/testsuite/g++.dg/eh/sighandle.C:30
(gdb) p frames
$6 = 5
(gdb)

frame count should be 4, not 5.  This patch skips signal frames when
unwinding shadow stack.

gcc/testsuite/

	PR libgcc/85334
	* g++.dg/torture/pr85334.C: New test.

libgcc/

	PR libgcc/85334
	* unwind-generic.h (_Unwind_Frames_Increment): New.
	* config/i386/shadow-stack-unwind.h (_Unwind_Frames_Increment):
	Likewise.
	* unwind.inc (_Unwind_RaiseException_Phase2): Increment frame
	count with _Unwind_Frames_Increment.
	(_Unwind_ForcedUnwind_Phase2): Likewise.

From-SVN: r259502