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

win32/tcc-win32.txt

@@ -54,7 +54,7 @@
         tcc examples/libtcc_test.c -I libtcc libtcc/libtcc.def
 
     - Compiling with MinGW:
-        gcc examples/libtcc_test.c -I libtcc libtcc.dll
+        gcc examples/libtcc_test.c -I libtcc libtcc.dll -o libtcc_test.exe
 
     - Compiling with MSVC:
         lib /def:libtcc\libtcc.def /out:libtcc.lib