panic-probe: use UDF instruction on nested panics

[japaric]

given this code:

use panic_probe as _;

struct S;

impl defmt::Format for S {
    fn format(&self, _: defmt::Formatter) {
        defmt::panic!()
    }
}

#[cortex_m_rt::entry]
fn main() -> ! {
    defmt::panic!("{}", S)
}

before this PR:

$ cargo rb panic
(HOST) INFO  flashing program (6.66 KiB)
(HOST) INFO  success!
────────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────────
(HOST) INFO  device halted without error

$ echo $?
0

after this PR:

(HOST) INFO  flashing program (6.64 KiB)
(HOST) INFO  success!
────────────────────────────────────────────────────────────────────────────────
────────────────────────────────────────────────────────────────────────────────
stack backtrace:
   0: HardFaultTrampoline
      <exception entry>
   1: lib::inline::__udf
        at ./asm/inline.rs:172:5
   2: __udf
        at ./asm/lib.rs:49:17
   3: cortex_m::asm::udf
        at /home/japaric/.cargo/registry/src/jackfan.us.kg-1ecc6299db9ec823/cortex-m-0.7.3/src/asm.rs:43:5
   4: rust_begin_unwind
        at /home/japaric/.cargo/git/checkouts/defmt-52fbd7917982cfac/76a0478/firmware/panic-probe/src/lib.rs:72:9
   5: core::panicking::panic_fmt
        at /rustc/c8dfcfe046a7680554bf4eb612bad840e7631c4b/library/core/src/panicking.rs:92:14
   6: core::panicking::panic
        at /rustc/c8dfcfe046a7680554bf4eb612bad840e7631c4b/library/core/src/panicking.rs:50:5
   7: <defmt_rtt::Logger as defmt::traits::Logger>::acquire
        at /home/japaric/.cargo/git/checkouts/defmt-52fbd7917982cfac/76a0478/firmware/defmt-rtt/src/lib.rs:45:13
(..)
(HOST) ERROR the program panicked

$ echo $?
6

[jonas-schievink]

bors r+

[bors]

Build succeeded:

• ci