combine rust and cargo templates

[classabbyamp]

@q66, wanted to get your opinion on these changes before I go further. I did my best to fold the cargo{,-bootstrap} templates into the rust{,-bootstrap} ones, preserving all patches and fixes. Tested successfully with a cross build x86_64 to aarch64. If this looks fine, I can continute working on doing some cleanups and update to 1.59 (although there is a PR for that already).

Testing the changes

• I tested the changes in this PR: YES

[ci skip]

[q66]

i don't see how this is supposed to cross compile, considering it does not really fix anything and rustbuild for cross stage 2 tools is not ready

[classabbyamp]

If stage 2 tools can't be crossed, then how was I able to successfully cross-build and use cargo using these templates?

[q66]

luck with a particular target combo? the tools get built with the right compiler, but they get the host's flags and environment (in rust build, they get build+host == the build env and target == the target env, while all correctly cross-compiled stuff should be build == build env and host+target == target env, but the logic for tools is hardcoded in the rustbuild code and when I looked into it it seemed extremely non-trivial to fix), which results in weirdness (in my cases, actual build failures; i tested this when doing the 1.57 update)