coredump: add COREDUMP_CODE field for signal reason

coredump: add COREDUMP_CODE field for signal reason

Introduce COREDUMP_CODE as a new captured field alongside the existing
COREDUMP_SIGNAL. While COREDUMP_SIGNAL identifies the signal number that
terminated the process, COREDUMP_CODE provides the reason the signal was sent.

For example, a process terminated by SIGSEGV due to invalid permissions would
produce COREDUMP_SIGNAL=11 and COREDUMP_CODE=2 (SEGV_ACCERR).

The kernel exposes coredump_code via pidfd starting with v7.1:
https://git.kernel.org/torvalds/c/701f7f4fbabbf4989ba6fbf033b160dd943221d5

System administrators can find both the signal and code in coredumpctl info:

$ coredumpctl info | grep Signal:
        Signal: 11 (SEGV) si_code: SEGV_MAPERR

Signed-off-by: Emanuele Rocca <emanuele.rocca@arm.com>