tests/posix/exec_sigmask: Make portable

The test was Linux-only, and didn't pass on at least mips64el due to a
different signal mask size.  The new version should be fully portable,
but will hang forever on failure.

Link: https://buildd.debian.org/status/fetch.php?pkg=bfs&arch=mips64el&ver=4.0.5-1&stamp=1737761227&raw=0

Author: tavianator

tests/posix/exec_sigmask.sh

@@ -1,4 +1,16 @@
 # Regression test: restore the signal mask after fork()
 
-test "$UNAME" = "Linux" || skip
-bfs_diff /proc/self/status -exec grep '^SigBlk:' {} +
+cd "$TEST"
+mkfifo p1 p2
+
+{
+    # Get the PID of `sh`
+    read -r pid <p1
+    # Send SIGTERM -- this will hang forever if signals are blocked
+    kill $pid
+} &
+
+# Write the `sh` PID to p1, then hang reading p2 until we're killed
+! invoke_bfs p1 -exec sh -c 'echo $$ >p1 && read -r _ <p2' {} + || fail
+
+wait