crossdirect: support rust

Right now, the rust compiler rustc is running in qemu. This is frustratingly slow, and hangs forever for @minlexx as reported in https://gitlab.com/postmarketOS/pmaports/merge_requests/841#note_265824821. So I looked into adding support to crossdirect.

Two hours later, I have a proof of concept. I can't compile squeekboard right now, because of an unrelated libdrm conflict (will try to figure that one out after posting this issue and related MRs), but it does cross compile a hello-world program successfully.

How it works:

• pmbootstrap build --arch=armhf runs the build in the buildroot_armhf chroot and mounts /native to point at buildroot_native, then runs the build with /native/usr/lib/crossdirect/armhf infront of PATH (as usually with crossdirect)
• rustc is installed in buildroot_native, if the package has rust in its build depends
• cargo runs in the buildroot_armhf
• a new /native/usr/lib/crossdirect/armhf/rustc shell script wrapper calls /native/usr/bin/rustc with options to make it cross compile to armhf
• one of the options sets the Clinker to point at another wrapper script, which calls gcc from the foreign arch chroot. This is necessary until #233 is resolved.

TODO:

• proof of concept, that works with hello-world-rust
• crosscompile squeekboard successfully for all supported arches still failing later in the build process, but having crossdirect support in rust (and proper caching of the rust dirs, see pmbootstrap MR) is already improving the situation, so I'm wrapping this up. I'll create a separate issue regarding squeekboard as soon as I find time; we should probably start splitting up the dependencies in own packages etc, and try to do the phosh upgrade first without the squeekboard upgrade if possible. -> #407
• merge pmaports MR (https://gitlab.com/postmarketOS/pmaports/merge_requests/845)
• add pmbootstrap test case (set up buildroot_armhf, replace /usr/bin/rustc with /bin/false, build hello-world-rust)
• merge pmbootstrap MR (!1850 (merged))

CC: @zhuowei