Don't export symbols or synthetic object files in Scratchbox

[direc85]

It evidently looks like the linker in Scratchbox / Sailfish SDK does not work with - nor need - the exported symbols and synthetic object files. Taking inspiration of the plain if-return just above the patch lines, I added a detection of SB2_RUST_TARGET_TRIPLE environment variable, which then results in those files not being created, and thus not being added to the linker file list.

With this patch Rust 1.75.0 compiles in Platform SDK shell (the hanging problem in the build target still exists - statx syscall is the main culprit) and installing the resulting packages into Sailfish SDK successfully lets me build Whisperfish 1.72 branch for i486, armv7hl and aarch64. The functionality of the armv7hl build was tested on-device with Jolla Phone, and it seems to work just fine (as a test I sent a message and checked the output on terminal).

This patch is kind of a hack, but then again, so are many of the other patches. Further improvements are most welcome!

[rubdos]

N.b., this would unfix this issue: rust-lang/rust#47384

[direc85]

As it turned out, the bug is fixable in environment (Whisperfish .spec in my use case) instead:

export TMPDIR=$(realpath "%{_sourcedir}/.tmp")
mkdir -p $TMPDIR

So, it's a bug somewhere in Scratchbox instead -- closing this PR as redundant (and potentially harmful).