x86 setup: fix constraints in segment accessor functions

Fix the operand constraints for the segment accessor functions,
{rd,wr}{fs,gs}*.  In particular, the 8-bit functions used "r"
constraints instead of "q" constraints.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>