riscv32: use gdb binary as a fallback

At least on my system (Fedora 42) the standard gdb binary is capable of
debugging riscv-qemu binaries (running inside qemu-system-riscv32).

targets/riscv32.json

@@ -12,5 +12,8 @@
 	"ldflags": [
 		"-melf32lriscv"
 	],
-	"gdb": ["gdb-multiarch"]
+	"gdb": [
+		"gdb-multiarch",
+		"gdb"
+	]
 }