Add a check for the VC++ 2012-style 64-bit path.
In VS2012 the path to the 64-bit tools has changed to
VCINSTALLDIR\bin\x86_amd64\cl.exe. This change will make the bootstrap
check there to see if it exists before falling back to the old amd64 path.
Merge pull request #504 from benvanik/master

Add a check for the VC++ 2012-style 64-bit path.