Xmake build flag parity with Cmake

[bitonality]

Did another parity pass on flags and I've identified three MSVC flags that are currently missing.

All build flavors:

/GR - Enable Run-Time Type Information

xxx_Shipping_xxx Flavors:

-DNDEBUG definition
/GL - Whole program optimization

• This shaves a couple megabytes off of the finished DLL... Xmake supports this concept across compilers by allowing us to set the build.optimization.lto policy for targets instead of manually adding flags-per-compiler.

/permissive- - Standards conformance

xxx_Debug_xxx Flavors:

/RTC1 - Runtime error checks

I'm not sure how important the non /GL flags are (since they've gone somewhat unnoticed so far), but I'm raising this issue for due diligence sake.