scripts/jobserver-exec: propagate child exit status

scripts/jobserver-exec: propagate child exit status

main() called JobserverExec().run() and discarded its return value,
then the script exited with the implicit status 0. As a result, any
Makefile that wired a build step through jobserver-exec saw the step
silently succeed even when the wrapped command had failed.

Two in-tree callers were affected:

  Documentation/devicetree/bindings/Makefile
    cmd_chk_style runs a python checker via jobserver-exec and uses
    "&& touch $@ || true" so failures leave the stamp file untouched
    and the next make rerun reports them again. The swallowed exit
    code made the stamp file get created even on failure, caching the
    failed run and hiding the reported issues until the inputs change.

  scripts/Makefile.vmlinux_o
    cmd_gen_initcalls_lds runs scripts/generate_initcall_order.pl via
    jobserver-exec; a perl failure was masked by the wrapper.

Return the subprocess exit code from main() and pass it to sys.exit()
so the wrapped command's status reaches make.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@oss.qualcomm.com>
Link: https://patch.msgid.link/660368ca16e2d3845577a9fd157d2f37f0e09e85.1779908995.git.daniel@makrotopia.org
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>