x86_64: Use __seg_fs qualifier to cast access to TCB in THREAD_GSCOPE_RESET_FLAG()

x86_64: Use __seg_fs qualifier to cast access to TCB in THREAD_GSCOPE_RESET_FLAG()

Use the __seg_fs named address space qualifier to cast access to the
gscope_flag in the TCB as a %fs: prefixed address.  This enables the
use of the "m" operand constraint, which informs the compiler about
memory access in the inline assembly.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: H.J.Lu <hjl.tools@gmail.com>
Cc: Florian Weimer <fweimer@redhat.com>
Cc: Carlos O'Donell <carlos@redhat.com>