[arm] __builtin_trap should emit udf not brk

```c
void foo () {
__builtin_trap();
}
```
emits with clang:
```asm
_foo:
brk #0x1
```
but with gcc:
```asm
_foo:
udf #255
```
This makes handling traps from ubsan in the linux kernel compiler dependent, which sucks.
https://lore.kernel.org/llvm/CAD++jLk549ZcSjpRN0G27i1C-n-k-dOpmgvfZBgmHQ5s0F6dJg@mail.gmail.com/

Start with the provided C reproducer and compare the ARM assembly emitted by Clang with GCC, focusing on how __builtin_trap is lowered. The change is complete when the ARM output uses udf #255 instead of brk #0x1, with coverage for the behavior in the compiler's relevant tests.

Written by the indexing model from the issue text.