The failure appeared after Bug 1510785 landed.
Log link: https://treeherder.mozilla.org/logviewer.html#?job_id=270408832&repo=autoland

Log snippet:
[task 2019-10-09T05:57:40.107Z] creating build/temp.linux-x86_64-2.7/psutil
[task 2019-10-09T05:57:40.107Z] x86_64-linux-gnu-gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fno-strict-aliasing -Wdate-time -D_FORTIFY_SOURCE=2 -g -fdebug-prefix-map=/build/python2.7-IbFBHb/python2.7-2.7.16=. -fstack-protector-strong -Wformat -Werror=format-security -fPIC -DPSUTIL_POSIX=1 -DPSUTIL_VERSION=543 -DPSUTIL_LINUX=1 -DPSUTIL_ETHTOOL_MISSING_TYPES=1 -I/usr/include/python2.7 -c psutil/_psutil_common.c -o build/temp.linux-x86_64-2.7/psutil/_psutil_common.o
[task 2019-10-09T05:57:40.107Z] unable to execute 'x86_64-linux-gnu-gcc': No such file or directory
[task 2019-10-09T05:57:40.107Z] error: command 'x86_64-linux-gnu-gcc' failed with exit status 1
[task 2019-10-09T05:57:40.107Z]
[task 2019-10-09T05:57:40.107Z] Error processing command. Ignoring because optional. (optional:setup.py:third_party/python/psutil:build_ext:--inplace)
[task 2019-10-09T05:57:40.107Z] Error processing command. Ignoring because optional. (optional:packages.txt:comm/build/virtualenv_packages.txt)
[task 2019-10-09T05:57:42.087Z] + for dir in a b
[task 2019-10-09T05:57:42.087Z] + cd /builds/worker/checkouts/gecko
[task 2019-10-09T05:57:42.087Z] + ./mach python toolkit/mozapps/installer/unpack.py --omnijar omni.ja /builds/worker/b
[task 2019-10-09T05:57:44.516Z] + for option in "$@"
[task 2019-10-09T05:57:44.516Z] + case "$option" in
[task 2019-10-09T05:57:44.516Z] + POST='exit 1'
..............................
[task 2019-10-09T05:57:51.801Z] + exit 1
[taskcluster 2019-10-09 05:58:20.381Z] === Task Finished ===
[taskcluster 2019-10-09 05:58:21.183Z] Unsuccessful task run with exit code: 1 completed in 102.159 seconds

I'm unsure why the XBL removal affected this. Any thoughts glandium?

See https://taskcluster-artifacts.net/DvKMur0FSGa5X-6YfIzv7g/0/public/diff.html

One of the input builds has XBL enabled and the other not. One is build-win64-aarch64-eme/opt, the other is build-win64-aarch64/opt.

Hi Brendan, any updates here?

I believe this will be fixed by https://treeherder.mozilla.org/#/jobs?repo=try&revision=3ce92da7abcb356ea149357abb4968e6f15a64be. As soon as that finishes I'll get review.

Thank you.

Fixed in bug 1587662.