Platform detection on MinGW+Windows

[Kriper1111]

In the xmake config script, there is a platform check performed when the build system is MinGW.

UnityDoorstop/xmake.lua

Lines 49 to 57 in 5607be4

	if is_plat("mingw") then
	add_shflags("-nostdlib", "-nolibc", {force=true})
	if is_arch("i386") then
	add_shflags("-e _DllEntry", "-Wl,--enable-stdcall-fixup", {force=true})
	elseif is_arch("x86_64") then
	add_shflags("-e DllEntry", {force=true})
	end
	end

When running on my Windows machine under MinGW, neither branch is being taken and the library produced has no valid entrypoint.

Here's what xmake show tells on my machine:

> tools\xmake\xmake show
The information of xmake:
    version: 2.6.1+HEAD.666dc5479
    host: windows/x64
    programdir: E:\Coding\UnityDoorstop\tools\xmake
    programfile: E:\Coding\UnityDoorstop\tools\xmake\xmake.exe
    globaldir: C:\Users\%username%\AppData\Local\.xmake
    tmpdir: C:\Users\%username%\AppData\Local\Temp\.xmake\240817
    workingdir: E:\Coding\UnityDoorstop
    packagedir: C:\Users\%username%\AppData\Local\.xmake\packages
    packagedir(cache): C:\Users\%username%\AppData\Local\.xmake\cache\packages\2408

The information of project:
    plat: mingw
    arch: x64
    mode: release
    buildir: build
    configdir: E:\Coding\UnityDoorstop\.xmake\windows\x64
    projectdir: E:\Coding\UnityDoorstop
    projectfile: E:\Coding\UnityDoorstop\xmake.lua

The supplied fix fix for that changes the condition on line 54 to is_arch("x64", "x86_64"), as also suggested by xmake docs.