(1) The output registers were not marked call-clobbered, even though
    they can be modified by called functions.
(2) The thread pointer was not marked reserved.
(3) R4-R6 are call-saved, but not saved by the prologue.  Rather than
    save them, mark them reserved so that we don't use them.

Signed-off-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>