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: 3cc31d79 ("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