accel/tcg: better handle memory constrained systems

accel/tcg: better handle memory constrained systems

It turns out there are some 64 bit systems that have relatively low
amounts of physical memory available to them (typically CI system).
Even with swapping available a 1GB translation buffer that fills up
can put the machine under increased memory pressure. Detect these low
memory situations and reduce tb_size appropriately.

Fixes: 600e17b2615 ("accel/tcg: increase default code gen buffer size for 64 bit")
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Robert Foley <robert.foley@linaro.org>
Cc: BALATON Zoltan <balaton@eik.bme.hu>
Cc: Christian Ehrhardt <christian.ehrhardt@canonical.com>
Message-Id: <20200724064509.331-7-alex.bennee@linaro.org>