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

tests/ui/invalid-compile-flags/crate-type-flag.rs

@@ -30,7 +30,7 @@
 //@[bin] check-pass
 
 //@[proc_dash_macro] ignore-wasm (proc-macro is not supported)
-//@[proc_dash_macro] compile-flags: --crate-type=proc-macro
+//@[proc_dash_macro] compile-flags: --crate-type=proc-macro -C panic=unwind
 //@[proc_dash_macro] check-pass
 
 //@ revisions: multivalue multivalue_combined