selftests/seccomp: sh: Fix register names

It looks like the seccomp selftests was never actually built for sh.
This fixes it, though I don't have an environment to do a runtime test
of it yet.

Fixes: 0bb605c2 ("sh: Add SECCOMP_FILTER")
Tested-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/lkml/a36d7b48-6598-1642-e403-0c77a86f416d@physik.fu-berlin.deSigned-off-by: Kees Cook <keescook@chromium.org>

tools/testing/selftests/seccomp/seccomp_bpf.c

@@ -1804,8 +1804,8 @@ TEST_F(TRACE_poke, getpid_runs_normally)
 #define SYSCALL_RET(_regs)	(_regs).a[(_regs).windowbase * 4 + 2]
 #elif defined(__sh__)
 # define ARCH_REGS		struct pt_regs
-# define SYSCALL_NUM(_regs)	(_regs).gpr[3]
-# define SYSCALL_RET(_regs)	(_regs).gpr[0]
+# define SYSCALL_NUM(_regs)	(_regs).regs[3]
+# define SYSCALL_RET(_regs)	(_regs).regs[0]
 #else
 # error "Do not know how to find your architecture's registers and syscalls"
 #endif