arm: automatically determine stack sizes

This is a big change that will determine the stack size for many
goroutines automatically. Functions that aren't recursive and don't call
function pointers can in many cases have an automatically determined
worst case stack size. This is useful, as the stack size is usually much
lower than the previous hardcoded default of 1024 bytes: somewhere
around 200-500 bytes is common.

A side effect of this change is that the default stack sizes (including
the stack size for other architectures such as AVR) can now be changed
in the config JSON file, making it tunable per application.

src/runtime/scheduler_cortexm.S

@@ -41,11 +41,7 @@ tinygo_getSystemStackPointer:
     .cfi_endproc
 .size tinygo_getSystemStackPointer, .-tinygo_getSystemStackPointer
 
-
-// switchToScheduler and switchToTask are also in the same section, to make sure
-// relative branches work.
-.section .text.tinygo_swapTask
-
+.section .text.tinygo_switchToScheduler
 .global  tinygo_switchToScheduler
 .type    tinygo_switchToScheduler, %function
 tinygo_switchToScheduler:
@@ -62,6 +58,7 @@ tinygo_switchToScheduler:
     .cfi_endproc
 .size tinygo_switchToScheduler, .-tinygo_switchToScheduler
 
+.section .text.tinygo_switchToTask
 .global  tinygo_switchToTask
 .type    tinygo_switchToTask, %function
 tinygo_switchToTask:
@@ -72,11 +69,11 @@ tinygo_switchToTask:
     // and then we can invoke swapTask.
     msr PSP, r0
 
-    // Continue executing in the swapTask function, which swaps the stack
-    // pointer.
+    b.n tinygo_swapTask
     .cfi_endproc
 .size tinygo_switchToTask, .-tinygo_switchToTask
 
+.section .text.tinygo_swapTask
 .global  tinygo_swapTask
 .type    tinygo_swapTask, %function
 tinygo_swapTask: