Add x64 platform to MSVS 10 solution and project files.

Manually update VC10 projects to have both Win32 and x64 architectures, use
"vc_x64" for the output directories and the DLL names under the latter.

Closes #15568.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75079 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775
Simplify and update wxMSW build instructions.

Keep the most important points, remove the obsolete stuff.

Also get rid of 99% of the contents of BuildSVN.txt which simply duplicated
the information from the other files.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/branches/WX_3_0_BRANCH@75080 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775