x86: introduce read_sregs() to allow storing to memory directly

x86: introduce read_sregs() to allow storing to memory directly

When storing all (data) segment registers in one go, prefer writing the
selector values directly to memory (as opposed to read_sreg()).

Also move the single register variant into the regs.h, dropping the
unecessary "volatile" from the asm() (there are no hidden side effects).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>