amd64: relax constraints in curthread and curpcb

This makes the compiler less likely to reload the content from %gs.

The 'P' modifier drops all synteax prefixes and 'n' constraint treats
input as a known at compilation time immediate integer.

Example reloading victim was spinlock_enter.

Stolen from: OpenBSD

Reported by: jtl
Reviewed by: kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D17615