rust: Fix Host multilib prop for rust_test modules.

NewRustTest will set MultilibBoth true for modules which target
devices, however modules which target both device and host
incorrectly sets MultilibBoth for the host variant. Host tests
can only produce the primary arch target. Therefore, add the
rustTestHostMultilib load hook to set MultilibFirst for the
host variant.

Test: m rust_test module with host_supported: true, check if x86 test
      produced.
Change-Id: I5b04b2a2d898823bcf326336d1d28c46945d47f5