Validate user supplied TARGET (#1941)

the build will now abort with an error message when an undefined build TARGET is named

Fixes #1938

Makefile.system

@@ -65,6 +65,7 @@ endif
 
 ifdef TARGET
 GETARCH_FLAGS := -DFORCE_$(TARGET)
+GETARCH_FLAGS += -DUSER_TARGET
 endif
 
 # Force fallbacks for 32bit

getarch.c

@@ -1068,6 +1068,10 @@ USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
 #ifndef FORCE
 
+#ifdef USER_TARGET
+#error "The TARGET specified on the command line or in Makefile.rule is not supported. Please choose a target from TargetList.txt"
+#endif
+
 #if defined(__powerpc__) || defined(__powerpc) || defined(powerpc) || \
     defined(__PPC__) || defined(PPC) || defined(_POWER) || defined(__POWERPC__)
 #ifndef POWER