[build system] Select llvm target

[Yamakaky]

Currently, llvm is built with all the possible targets including Mips, ARM... For development, we can speed the build by enabling only the native target (x86 or x86_64). Maybe it would also speed the build of rustc?

[japaric]

For development, ...

FWIW, for development I use ccache (pass --enable-ccache to configure) and building LLVM takes one minute or less on a "fresh" bootstrap. (I usually work with several branches at the same time; one docker container per branch and all of them reuse the same ccache directory)

we can speed the build by enabling only the native target

Just please don't change the defaullt. I'd be very upset if I spent several minutes building stage1 rustc just to learn that it can't cross compile anything.

[Yamakaky]

yeah, of course.

[xen0n]

This is such a niche use case, to the point that it's arguably already moot by use of ccache, if the sole purpose is to reduce build time. However there may be some other advantages, maybe space savings for use on some really storage-constrained systems? I don't know, it's trivial to make the list of targets configurable but the maintenance and support burden is real.

What do the Rust team think? cc @alexcrichton

[alexcrichton]

Seems fine to me to add an option for this! We won't do it on the bots, but that doesn't mean it can't exist.