[CMake] use same generator for compiler and stdlib builds

[edymtt]

This will support configurations where ninja is not the search path, and will avoid problems that can arise from the stdlib build detecting and using a different generator.

Addresses rdar://142268599

[edymtt]

@swift-ci please test

[compnerd]

I believe that the CMAKE_GENERATOR is implicitly set to the same one, so this should be sufficient.

[edymtt]

According to the Cmake documentation, it seems it is the other way around -- that is, if not set explicitly CMAKE_MAKE_PROGRAM is set to the value of CMAKE_GENERATOR.

This seems to be corroborated by what I experienced at desk when trying to build without having ninja in my search path -- CMAKE_GENERATOR=ninja was inherited from the "parent project", but CMAKE_MAKE_PROGRAM=<path to local ninja build> was not and thus was set to the same value as CMAKE_GENERATOR, and this caused the configuration of the standalone stdlib to fail