selftests/powerpc: Fix out of bounds access in TM signal test

Gcc helpfully points out that we're accessing past the end of the gprs
array:

  tm-signal-msr-resv.c: In function 'signal_usr1':
  tm-signal-msr-resv.c:43:37: error: array subscript is above array bounds [-Werror=array-bounds]
    ucp->uc_mcontext.regs->gpr[PT_MSR] |= (7ULL);

We haven't noticed previously because -flto was hiding it somehow.

The code is confused, PT_MSR isn't a gpr, instead it's in
uc_regs->gregs, so fix it.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

tools/testing/selftests/powerpc/tm/tm-signal-msr-resv.c

@@ -40,7 +40,7 @@ void signal_usr1(int signum, siginfo_t *info, void *uc)
 #ifdef __powerpc64__
 	ucp->uc_mcontext.gp_regs[PT_MSR] |= (7ULL << 32);
 #else
-	ucp->uc_mcontext.regs->gpr[PT_MSR] |= (7ULL);
+	ucp->uc_mcontext.uc_regs->gregs[PT_MSR] |= (7ULL);
 #endif
 	/* Should segv on return becuase of invalid context */
 	segv_expected = 1;