Add os_get_available_system_memory() derived from
src/intel/vulkan/anv_device.c get_available_system_memory()
Fixes: b80930a6fe ("anv: add support for VK_EXT_memory_budget")
Signed-off-by: Jonathan Gray <jsg@jsg.id.au>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6517>