Function xsave_size_for_info 

fn xsave_size_for_info(info: &CpuInfo, xcr0: u64) -> usize

Compute the XSAVE area size needed for a given XCR0 mask directly from a CpuInfo, without consulting the global cache (host_uses_xsave/host). This is what init() must use, because the cache is not yet published when init() runs : otherwise AVX/AVX-512 would be given a 512-byte area.