cortexm: move SemihostingCall impl to the right asm file

This is the more correct location. The targets/cortex-m.s file is really
just for qemu, not for Cortex-M in general.

src/device/arm/cortexm.s

@@ -19,3 +19,12 @@ HardFault_Handler:
 
     // Continue handling this error in Go.
     bl handleHardFault
+
+// This is a convenience function for semihosting support.
+// At some point, this should be replaced by inline assembly.
+.section .text.SemihostingCall
+.global  SemihostingCall
+.type    SemihostingCall, %function
+SemihostingCall:
+    bkpt 0xab
+    bx   lr

targets/cortex-m.s

@@ -3,15 +3,6 @@
 
 .syntax unified
 
-// This is a convenience function for QEMU semihosting support.
-// At some point, this should be replaced by inline assembly.
-.section .text.SemihostingCall
-.global  SemihostingCall
-.type    SemihostingCall, %function
-SemihostingCall:
-    bkpt 0xab
-    bx   lr
-
 // This is the default handler for interrupts, if triggered but not defined.
 .section .text.Default_Handler
 .global  Default_Handler