The patch "fix-cpuid-on-abi-x86_32.patch" is being applied on the arm platform when it really shouldn't be. This path introduces an #include statement for cpuid.h. Cpuid.h doesn't live on arm, and so this causes the compilation to fail. If this patch is not applied, compilation will then succeed.
Dion, thank you for the analysis and the report.
commit 7ae1540f67fcd915fb3b5753c7a33a23c8467e09 (HEAD -> master, origin/master, origin/HEAD) Author: Alexis Ballier <aballier@gentoo.org> Date: Tue Aug 1 14:33:36 2017 +0200 media-libs/openexr: Update openexr-2.2.0-fix-cpuid-on-abi_x86_32.patch to netbsd one, current one causes build failures on non x86 arches. Bug #626760 thx