Explicitly build proc macro test with panic=unwind
Fuchsia explicitly builds rust and all rust targets with `-C panic=abort` to minimize code generation size. However, when compiling a proc-macro with this setting it can cause a warning to be emitted, which breaks `tests/ui/invalid-compile-flags/crate-type-flag.rs`. This hasn't been a problem in the past for us since we compile our proc macros on host, rather than inside Fuchsia.
This attempts to fix the issue by explicitly requiring that we're using the unwinder when compiling this test to avoid the warning being emitted.
Fixes#135223
84910fc3cd