uucore: support signals feature for haiku

[xtqqczze]

Add Haiku to the platforms supporting the signals feature and define its signal set.

The feature builds successfully with Clippy for Haiku:

$ RUSTC_BOOTSTRAP=1 cargo clippy -q -Zbuild-std -p uucore --features
signals --all-targets --target x86_64-unknown-haiku >/dev/null 2>&1;
echo $?
0

The signal list was generated from Haiku’s posix/signal.h.

curl -fsSL
'https://raw.githubusercontent.com/haiku/haiku/refs/heads/master/headers/posix/signal.h'
|
awk '/^[[:space:]]*#define[[:space:]]+SIG[A-Z0-9_]+[[:space:]]+[0-9]+/ {
    name=$2
    sub(/^SIG/, "", name)
    sigs[$3]=name
}
END {
    print "EXIT"
    for (i=1; i<=32; i++) print sigs[i]
}'

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/cut/bounded-memory (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/date/date-locale-hour (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/tail/pipe-f is now passing!
Skip an intermittent issue tests/pr/bounded-memory (was skipped on 'main', now failing)