#2152 Turn on /Qspectre to enable Spectre mitigations

[Adam Gross]

Would like to request that 7-zip turn on /Qspectre as documented in https://docs.microsoft.com/en-us/cpp/build/reference/qspectre (requires VS2017U5 or later if dynamically linking to CRT, VS2017 U7 if statically linking to CRT). To validate this, you can run BinSkim ( https://www.nuget.org/packages/Microsoft.CodeAnalysis.BinSkim/ ) with --verbose on a compiled binary and pdb to verify that rule BA2024 passes.