i386: properly define BR2_ARCH for all x86 subarchitectures

i386: properly define BR2_ARCH for all x86 subarchitectures

We were not defining BR2_ARCH for two of the x86 subarchitecture,
which means the architecture part of the GNU_TARGET_NAME tuple was
empty, leading to weird build failures.

This commit fixes that by defining the appropriate values.

Fixes bug #8861.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>