add support for 32-bit ARM on Windows (#477)

* add support for 32-bit ARM on Windows

* fix mismatched brace in appveyor.yml

* remove arm platform from appveyor.yml for now

* fix arm build

* fix typo

* fix assembler names

* try Visual Studio 2017

* add windows arm32 to .appveyor.yml

* update README.md

Makefile.am

@@ -82,6 +82,7 @@ EXTRA_libffi_la_SOURCES = \
 	src/alpha/ffi.c src/alpha/osf.S					\
 	src/arc/ffi.c src/arc/arcompact.S				\
 	src/arm/ffi.c src/arm/sysv.S					\
+	src/arm/ffi.c src/arm/sysv_msvc_arm32.S			\
 	src/avr32/ffi.c src/avr32/sysv.S				\
 	src/bfin/ffi.c src/bfin/sysv.S					\
 	src/cris/ffi.c src/cris/sysv.S					\