Maintainer updates for testing, gitlab, gdbstub and plugins: - add dtc package to openbsd VMs - use -fno-stack-protector for non-stdlib tests - split alpha and sh4 compilers into legacy image - harmonise other compilers into debian-all-test-cross - fix NULL check in gdb_regs - fix memleak in semihosting - remove unused parameter in plugin code - fix fd leak in lockstep plugin