Investigate cross-compile dist tool flow incorrectly building {host,build} compilers

[jieyouxu]

./x dist --stage=2 --host=... --target=... does the wrong thing:

• Compiles build compiler from stage0 to stage1 (ok)
• Compiles host compiler from stage1 to stage2(host) (ok)
• Compiles host compiler from stage0 to stage1(host) (wrong, why is it doing this?)
• Compiles build compiler from stage1 to stage2 (wrong, why is it doing this?)

./x dist --stage=1 --host=... --target=... also does the wrong thing:

• Compiles host compiler from stage0 to stage1(host) (wrong, should be from stage1)
• Compiles build compiler from stage0 to stage1 (ok)
• Compiles host compiler from stage1 to stage2(host) (ok)

My expectations are:

• Cross compiling versus not cross compiling should not require different stage settings. But you seem to be saying that we need to use different commands for those?
• When compiling (either cross or not), it should:
  • build stage1 build compiler from stage0(build)
  • build stage2 host compiler from stage1(build)

From what I can tell, it seems to work correctly when not cross compiling.

Originally posted by @ehuss in #137476 (comment)

[jieyouxu]

(Assigned Onur too since you said you wanted to look into it)