kbuild: uapi: handle UML in architecture-specific exclusion lists

kbuild: uapi: handle UML in architecture-specific exclusion lists

When building User Mode Linux SRCARCH is set to 'um', while the actual
underlying architecture is provided in HEADER_ARCH.

Allow the exclusion lists to work on UML by comparing against
HEADER_ARCH when that is available.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://patch.msgid.link/20260316-kbuild-uapi-c-v2-3-35d6d0ed863f@weissschuh.net
Signed-off-by: Nicolas Schier <nsc@kernel.org>