asmjs: Enable CRT file support.

include/bx/config.h

@@ -24,6 +24,7 @@
 #	define BX_CONFIG_CRT_FILE_READER_WRITER (0 \
 				|| BX_PLATFORM_ANDROID \
 				|| BX_PLATFORM_FREEBSD \
+				|| BX_PLATFORM_EMSCRIPTEN \
 				|| BX_PLATFORM_IOS \
 				|| BX_PLATFORM_LINUX \
 				|| BX_PLATFORM_OSX \