Build should work with VS 2013 express for 64bit builds

Visual Studio 2013 express edition does not come with the native 64bit compilers which we assume in configure. Instead it comes with a 32bit cross compilation setup. The only real difference seems to be the location of files, most notably the vcvars*.bat. Nir Lisker on build-dev has demonstrated that by changing which vcvars*.bat we look for, he was able to successfully build using express edition: http://mail.openjdk.java.net/pipermail/build-dev/2018-January/020584.html

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This is what I did in toolchain_windows.m4:

    if test "x$OPENJDK_TARGET_CPU_BITS" = x32; then
       VCVARSFILE="vc/bin/vcvars32.bat"
    else
# VCVARSFILE="vc/bin/amd64/vcvars64.bat"
       VCVARSFILE="vc/bin/x86_amd64/vcvarsx86_amd64.bat"
    fi
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We should update configure to at least accept this alternate path to vcvars*.bat.