configure: arm: detect default thumb state of compiler

This makes configure honour the compiler default if thumb is not
explicitly enabled or disabled.
Signed-off-by: Mans Rullgard <mans@mansr.com>

configure

@@ -3092,6 +3092,7 @@ if enabled alpha; then
 
 elif enabled arm; then
 
+    check_cpp_condition stddef.h "defined __thumb__" && enable_weak thumb
     enabled thumb && check_cflags -mthumb || check_cflags -marm
     nogas=die