Catch up to linux-user's 8f67b9c694, 13c1339755, 2f7828b572, and
95059f9c31 by Richard Henderson which made reserved_va a run-time
calculation, defaulting to nothing except in the case of 64-bit host
32-bit target. Also include the adjustment of the comment heading that
work submitted in the same patch stream. Since this is a direct copy,
squash it into one patch rather than follow the Linux evolution since
breaking this down further at this point doesn't make sense for this
"new code".
Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
cb4c259052