arch/arm: add THUMB interworking support

Add config option to build wtarget code with THUMB interworking.
This is used to build the C library as well as all other code
that runs on the target.

config/arch/arm.in

@@ -38,6 +38,20 @@ config ARCH_ARM_MODE_THUMB
 
 endchoice
 
+config ARCH_ARM_INTERWORKING
+    bool
+    prompt "Use Thumb-interworking (READ HELP)"
+    default n
+    depends on EXPERIMENTAL
+    help
+      Excerpt from the gcc manual:
+      
+      > Generate code which supports calling between the ARM and Thumb
+      > instruction sets. Without this option the two instruction sets
+      > cannot be reliably used inside one program. The default is
+      > [not to use interwork], since slightly larger code is generated
+      > when [interwork] is specified.
+
 config ARCH_ARM_EABI
     bool
     prompt "Use EABI"

scripts/build/arch/arm.sh

@@ -20,4 +20,8 @@ CT_DoArchTupleValues() {
 #            CT_ARCH_TARGET_CFLAGS="-mthumb"
             ;;
     esac
+
+    if [ "${CT_ARCH_ARM_INTERWORKING}" = "y" ]; then
+        CT_ARCH_TARGET_CFLAGS+=" -mthumb-interwork"
+    fi
 }