runqemu: set memory size to 256M for most qemu machines

runqemu: set memory size to 256M for most qemu machines

Set memory size to 256M for qemuarm, qemux86, qemux86-64, qemumips,
qemumips64, and qemuppc.

This allows the smart automated tests to run on machines with a GUI
environment (such as Sato) running at the same time, for which 128M is
too limiting. Setting this in runqemu allows users manually using
runqemu to avoid the same out-of-memory issues under similar conditions
using smart, on-target compilation or other uses.

Fixes [YOCTO #5045].

(From OE-Core rev: fe5dfdece98692f8fa731c8d11c907a272266ea5)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>