riscv: fix offset in 64bit scheduler

Also keep common start.S file for 64 and 32 bit architectures.

targets/riscv.json

@@ -16,5 +16,8 @@
 	"ldflags": [
 		"--gc-sections"
 	],
+	"extra-files": [
+		"src/device/riscv/start.S"
+	],
 	"gdb": "riscv64-unknown-elf-gdb"
 }

targets/riscv32.json

@@ -12,6 +12,6 @@
 	],
 	"extra-files": [
 		"src/runtime/scheduler_tinygoriscv.S",
-		"src/device/riscv/start.S"
+		"src/device/riscv/handleinterrupt32.S"
 	]
 }

targets/riscv64.json

@@ -12,6 +12,6 @@
 	],
 	"extra-files": [
 		"src/runtime/scheduler_tinygoriscv64.S",
-		"src/device/riscv/start64.S"
+		"src/device/riscv/handleinterrupt64.S"
 	]
 }