linux-user: Rewrite non-fixed probe_guest_base

linux-user: Rewrite non-fixed probe_guest_base

Use pgb_addr_set to probe for all of the guest addresses,
not just the main executable.  Handle the identity map
specially and separately from the search.

If /proc/self/maps is available, utilize the full power
of the interval tree search, rather than a linear search
through the address list.

If /proc/self/maps is not available, increase the skip
between probes so that we do not probe every single page
of the host address space.  Choose 1 MiB for 32-bit hosts
(max 4k probes) and 1 GiB for 64-bit hosts (possibly a
large number of probes, but the large step makes it more
likely to find empty space quicker).

Tested-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>