Workaround failing build_unix_debug

[mmilata]

Something needs the symbol even though we don't do unwinding (I think):

/bin/ld: build/unix/rust/debug/libtrezor_lib.a(core-05898138a596088a.core.4cbccd39-cgu.0.rcgu.o):(.data.DW.ref.rust_eh_personality[DW.ref.rust_eh_personality]+0x0): undefined reference to `rust_eh_personality'

Defining one that does nothing seems to avoid the problem, not sure what happens when it actually gets called. Needs nightly rustc. Possibly related: https://github.com/rust-lang/rust/issues/56152, https://docs.rust-embedded.org/embedonomicon/smallest-no-std.html#eh_personality