linux-user: expand reserved brk space for 64bit guests

A recent change to fix commpage allocation issues on 32bit hosts
revealed another intermittent issue on s390x. The root cause was the
headroom we give for the brk space wasn't enough causing the guest to
attempt to map something on top of QEMUs own pages. We do not
currently do anything to protect from this (see #555).

By inspection the brk mmap moves around and top of the address range
has been measured as far as 19Mb away from the top of the binary. As
we chose a smallish number to keep 32bit on 32 bit feasible we only
increase the gap for 64 bit guests. This does mean that 64-on-32
static binaries are more likely to fail to find a hole in the address
space but that is hopefully a fairly rare situation.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20220113165550.4184455-1-alex.bennee@linaro.org>

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