all: rename assembly files to .S extension

The Go tools only consider lowercase .s files to be assembly files. By
renaming these to uppercase .S files they won't be discovered by the Go
toolchain and listed as the SFiles to be assembled.

There is a difference between .s and .S: only uppercase .S will be
passed through the preprocessor. Doing that is normally safe, and
definitely safe in the case of these files.

targets/cortex-m.json

@@ -22,7 +22,7 @@
 		"--gc-sections"
 	],
 	"extra-files": [
-		"src/device/arm/cortexm.s",
+		"src/device/arm/cortexm.S",
 		"src/internal/task/task_stack_cortexm.S",
 		"src/runtime/asm_arm.S"
 	],

targets/nintendoswitch.json

@@ -30,6 +30,6 @@
     "targets/nintendoswitch.s",
     "src/internal/task/task_stack_arm64.S",
     "src/runtime/asm_arm64.S",
-    "src/runtime/runtime_nintendoswitch.s"
+    "src/runtime/runtime_nintendoswitch.S"
   ]
 }