gdb: enable to specify multiple candidates for gdb

targets/avr.json

@@ -19,5 +19,5 @@
 		"src/internal/task/task_stack_avr.S",
 		"src/runtime/gc_avr.S"
 	],
-	"gdb": "avr-gdb"
+	"gdb": ["avr-gdb"]
 }

targets/cortex-m.json

@@ -28,5 +28,5 @@
 		"src/internal/task/task_stack_cortexm.S",
 		"src/runtime/gc_arm.S"
 	],
-	"gdb": "gdb-multiarch"
+	"gdb": ["gdb-multiarch", "arm-none-eabi-gdb"]
 }

targets/cortex-m7.json

@@ -4,6 +4,5 @@
 	"cflags": [
 		"--target=armv7em-none-eabi",
 		"-Qunused-arguments"
-	],
-	"gdb": "arm-none-eabi-gdb"
+	]
 }

targets/gameboy-advance.json

@@ -27,6 +27,6 @@
 		"targets/gameboy-advance.s",
 		"src/runtime/gc_arm.S"
 	],
-	"gdb": "gdb-multiarch",
+	"gdb": ["gdb-multiarch"],
 	"emulator": ["mgba", "-3"]
 }

targets/riscv.json

@@ -21,5 +21,5 @@
 		"src/runtime/gc_riscv.S",
 		"src/device/riscv/handleinterrupt.S"
 	],
-	"gdb": "riscv64-unknown-elf-gdb"
+	"gdb": ["riscv64-unknown-elf-gdb"]
 }