Use WTF_CPU_UNKNOWN when building for riscv64

Use WTF_CPU_UNKNOWN when building for riscv64

Bug: https://bugs.webkit.org/show_bug.cgi?id=271371
Forwarded: no

WebKitGTK doesn't build on riscv64 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-riscv64.patch