gnu-toolchain: Fix cross-building case

We need to use a host-arches conditional.
16 jobs for sam/buildstream-autoconvert in 52 minutes and 13 seconds (queued for 5 seconds)
Status Job ID Name Coverage
  Convert
passed #30107228
convert

00:05:13

 
  Build System
passed #30107229
build-build-system

00:00:40

 
  All Systems
passed #30107230
build-base-system

00:00:33

passed #30107231
build-devel-system

00:00:38

passed #30107233
build-genivi-demo-platform-system

00:04:53

passed #30107234
build-gnome-system

00:10:06

passed #30107235
build-ivi-demo-platform-system

00:08:41

passed #30107236
build-minimal-system

00:00:28

passed #30107237
build-openstack-system

00:06:36

passed #30107238
build-trove-system

00:03:13

passed #30107240
build-weston-qt5-system

00:00:43

passed #30107239
build-weston-system

00:00:35

 
  Cross Sysroots
failed #30107242
build-sysroot-armv8b64

00:26:13

failed #30107243
build-sysroot-armv8l64

00:26:56

failed #30107244
build-sysroot-ppc64b

00:27:39

failed #30107245
build-sysroot-ppc64l

00:35:48

 
Name Stage Failure
failed
build-sysroot-ppc64l Cross Sysroots There is an unknown failure, please try again

Pipeline Summary
Total: 18
Session: 18
Pull Queue: processed 0, skipped 18, failed 0
Fetch Queue: processed 14, skipped 4, failed 0
Build Queue: processed 8, skipped 0, failed 1
Push Queue: processed 8, skipped 0, failed 0
ERROR: Job failed: exit code 1
failed
build-sysroot-armv8b64 Cross Sysroots There is an unknown failure, please try again

Pipeline Summary
Total: 18
Session: 18
Pull Queue: processed 0, skipped 18, failed 0
Fetch Queue: processed 14, skipped 4, failed 0
Build Queue: processed 8, skipped 0, failed 1
Push Queue: processed 8, skipped 0, failed 0
ERROR: Job failed: exit code 1
failed
build-sysroot-armv8l64 Cross Sysroots There is an unknown failure, please try again

Pipeline Summary
Total: 18
Session: 18
Pull Queue: processed 0, skipped 18, failed 0
Fetch Queue: processed 14, skipped 4, failed 0
Build Queue: processed 8, skipped 0, failed 1
Push Queue: processed 8, skipped 0, failed 0
ERROR: Job failed: exit code 1
failed
build-sysroot-ppc64b Cross Sysroots There is an unknown failure, please try again

Pipeline Summary
Total: 18
Session: 18
Pull Queue: processed 0, skipped 18, failed 0
Fetch Queue: processed 14, skipped 4, failed 0
Build Queue: processed 8, skipped 0, failed 1
Push Queue: processed 8, skipped 0, failed 0
ERROR: Job failed: exit code 1