libbpf: Fix riscv register names

riscv registers are accessed via struct user_regs_struct, not struct
pt_regs. The program counter member in this struct is called pc, not
epc. The frame pointer is called s0, not fp.

Fixes: 3cc31d7940 ("libbpf: Normalize PT_REGS_xxx() macro definitions")
Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220209021745.2215452-6-iii@linux.ibm.com

tools/lib/bpf/bpf_tracing.h

@@ -210,10 +210,10 @@
 #define __PT_PARM4_REG a3
 #define __PT_PARM5_REG a4
 #define __PT_RET_REG ra
-#define __PT_FP_REG fp
+#define __PT_FP_REG s0
 #define __PT_RC_REG a5
 #define __PT_SP_REG sp
-#define __PT_IP_REG epc
+#define __PT_IP_REG pc
 
 #endif