Support building for Fuchsia.

This CL adds configs for the arm64 and x64 fuchsia
device targets, sets up the necessary linker flags,
and disables some functionality that is not currently
supported on Fuchsia.

Bug: 119831161
Test: Compile walleye, internal validation against
fuchsia_arm64-eng and fuchsia_x86_64-eng.
Change-Id: I2881b99d2e3a1995e2d8c00a2d86ee101a972c94