linux-user/arm: Deliver SIGTRAP for UDF patterns used as breakpoints

linux-user/arm: Deliver SIGTRAP for UDF patterns used as breakpoints

The Linux kernel doesn't use the official bkpt insn for breakpoints;
instead it uses three instructions in the guaranteed-to-UNDEF space,
and generates SIGTRAP for these rather than the SIGILL that most
UNDEF insns generate:

https://elixir.bootlin.com/linux/v5.9.8/source/arch/arm/kernel/ptrace.c#L197

Make QEMU treat these insns specially too.  The main benefit of this
is that if you're running a debugger on a guest program that runs
into a GCC __builtin_trap() or LLVM "trap because execution should
never reach here" then you'll get the expected signal rather than a
SIGILL.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20201117155634.6924-1-peter.maydell@linaro.org