selftests/bpf: Define SYS_PREFIX for riscv

SYS_PREFIX was missing for a RISC-V, which made a couple of kprobe
tests fail.

Add missing SYS_PREFIX for RISC-V.

Fixes: 08d0ce30 ("riscv: Implement syscall wrappers")
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Link: https://lore.kernel.org/bpf/20231004110905.49024-3-bjorn@kernel.org

tools/testing/selftests/bpf/progs/bpf_misc.h

@@ -99,6 +99,9 @@
 #elif defined(__TARGET_ARCH_arm64)
 #define SYSCALL_WRAPPER 1
 #define SYS_PREFIX "__arm64_"
+#elif defined(__TARGET_ARCH_riscv)
+#define SYSCALL_WRAPPER 1
+#define SYS_PREFIX "__riscv_"
 #else
 #define SYSCALL_WRAPPER 0
 #define SYS_PREFIX "__se_"