add ARDUINO_ARCH_CH32 (openwch#116)

Author: hathach

platform.txt

@@ -89,7 +89,7 @@ build.pid=0
 
 # Build information's     
 #####################-D{build.board} -DARDUINO_ARCH_{build.arch} -DBOARD_NAME="{build.board}" -DVARIANT_H="{build.variant_h}"
-build.info.flags=-D{build.series} -DARDUINO={runtime.ide.version} -D{build.chip} -DVARIANT_H="{build.variant_h}"
+build.info.flags=-DARDUINO_ARCH_CH32 -D{build.series} -DARDUINO={runtime.ide.version} -D{build.chip} -DVARIANT_H="{build.variant_h}"
 
 # Defaults config
 build.xSerial=