Use WTF_CPU_UNKNOWN when building for X32

Use WTF_CPU_UNKNOWN when building for X32

Forwarded: no

WebKitGTK doesn't build on X32 even with the JIT disabled.
Treating the CPU as unknown is perhaps a bit severe, but it allows us
to get the build done until someone steps up to maintain this
properly.
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Gbp-Pq: Name fix-ftbfs-x32.patch