util: add phys_memory.h for all memory measurements
Move the os_get_total_physical_memory function from gallium to util, simplify it (all BSDs support sysconf I'm pretty sure, no need for sysctl), use it in anv, i915, i965, iris and swrast.
This was also acknowledged like a year ago, but needed more testing. Hopefully it will get more testing on GitLab :)
The extraction into a separate header and usage in Intel stuff is pretty trivial, but the reshuffling of the function body affects various platforms. Would be nice to have it tested on macOS and the more exotic platforms (Windows, Cygwin, Haiku etc)