qemu: rename assembly file

The name was cortex-m.s which looks like it is a generic assembly file
for all cortex-m targets. However, it really is only for qemu
simulation, because every chip has a slightly different interrupt vector
table.

targets/qemu.json

@@ -10,7 +10,7 @@
 		"-T", "targets/lm3s6965.ld"
 	],
 	"extra-files": [
-		"targets/cortex-m.s"
+		"targets/qemu.s"
 	],
 	"emulator": ["qemu-system-arm", "-machine", "lm3s6965evb", "-semihosting", "-nographic", "-kernel"]
 }