Don't use -mapcs-frame with Clang

build/jam/ArchitectureRules

@@ -43,8 +43,13 @@ rule ArchitectureSetup architecture
 
 	local cpu = $(HAIKU_CPU_$(architecture)) ;
 	if $(cpu) = arm {
-		# For stackcrawls
-		gccBaseFlags += -mapcs-frame ;
+		if $(HAIKU_CC_IS_CLANG_$(architecture)) = 1 {
+			# Clang only defines __ARMEL__ when using -target arm-haiku-unknown
+			gccBaseFlags += -D__ARM__=1 ;
+		} else {
+			# For stackcrawls - not supported by Clang
+			gccBaseFlags += -mapcs-frame ;
+		}
 	}
 
 	# activating graphite optimizations