win32/64: msys2 support

Support ./configure && make under msys2 (a new msys fork)
on win32 and win64.

Get rid of CONFIG_WIN64 make-var. (On windows, WIN32 in
general is used for both 32 and 64 bit platforms)

Also:
- cleanup win32/build-tcc.bat
- adjust win32/(doc/)tcc-win32.tx

configure

@@ -50,6 +50,7 @@ cpu=
 targetos=`uname`
 case $targetos in
   MINGW32*)  mingw32=yes;;
+  MSYS*)     mingw32=yes;;
   DragonFly) noldl=yes;;
   OpenBSD)   noldl=yes;;
   FreeBSD)   noldl=yes;;