When the build.zig logic to build libzigstage2 was converted to a cmake
command, it neglected to use baseline CPU features rather than compiling
with native features.

This adds a hard coded flag `-mcpu=baseline` which can be used to detect
the native target, but mark it as non-native so that it does not get the
CPU features specific to the host used to compile libzigstage2.

Full `-mcpu` support is happening in the upcoming pull request #4509,
and so this "quick fix" will be cleaned up in that branch, before it is
merged to master.

closes #4506