libbpf: Fix invalid return address register in s390

There is currently an invalid register mapping in the s390 return
address register. As the manual[1] states, the return address can be
found at r14. In bpf_tracing.h, the s390 registers were named
gprs(general purpose registers). This commit fixes the problem by
correcting the mistyped mapping.

[1]: https://uclibc.org/docs/psABI-s390x.pdf#page=14

Fixes: 3cc31d79 ("libbpf: Normalize PT_REGS_xxx() macro definitions")
Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20221224071527.2292-7-danieltimlee@gmail.com

tools/lib/bpf/bpf_tracing.h

@@ -137,7 +137,7 @@ struct pt_regs___s390 {
 #define __PT_PARM3_REG gprs[4]
 #define __PT_PARM4_REG gprs[5]
 #define __PT_PARM5_REG gprs[6]
-#define __PT_RET_REG grps[14]
+#define __PT_RET_REG gprs[14]
 #define __PT_FP_REG gprs[11]	/* Works only with CONFIG_FRAME_POINTER */
 #define __PT_RC_REG gprs[2]
 #define __PT_SP_REG gprs[15]