When calling 'update.py fuchsia', will now build and generate for x64
and arm64. Note that David Turner (digit@) had already manually created
the configs by copying and modifying the Linux ones as part of this
change:
https://swiftshader-review.googlesource.com/c/SwiftShader/+/43851

After running update.py, I kept the Fuchsia-specific changes that were
made mainly to config.h. The diffs mainly remove unsupported targets,
and also add arm64, which was missing. Note that arm64 configs were
present in the LLVM 7 Fuchsia configs, so likely it was just an
oversight or temporary thing.

Tested by succesfully building Chromium for Fuchsia, both targeting x64
and arm64.

Bug: b/152339534
Change-Id: I15258f2a134808d3c394fad028e1a18acbd081f7
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/46489Tested-by: Antonio Maiorano <amaiorano@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Result: kokoro <noreply+kokoro@google.com>