Fix building the crossdacs with ninja.

[jkoritzinsky]

This will fix running ./build clr -ninja from the command line to build as well as enable us to use Ninja for building the whole product.

We were accidentally setting __BuildNative=1 here from when the -ninja option was created based on the usenmakemakefiles option.

Contributes to #44761

[hoyosjs]

cc: @sdmaclea

[sdmaclea]

/cc @vitek-karas

[hoyosjs]

@sdmaclea are these well known?

2020-12-15T20:14:26.0787787Z [24/477] Building C object pal\src\libunwind\src\CMakeFiles\libunwind_xdac.dir\x86_64\Gapply_reg_state.c.obj
2020-12-15T20:14:26.0788859Z cl : Command line warning D9014 : invalid value '9025' for '/wd'; assuming '5999'

[sdmaclea]

I think that was introduced by #45902

runtime/src/coreclr/pal/src/libunwind/CMakeLists.txt

Line 151 in 1cdafd2

add_compile_options(-wd9025) # overriding '/TP' with '/TC'

I wanted to suppress the D9025 warning reported in #45763. Not sure if it worked at all because that type of error was scattered everywhere with the VS Preview dogfood compilers. The line could be removed if it is not helpful.

[ghost]

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