x86/speculation: reformatting RSB overwrite macro

x86/speculation: reformatting RSB overwrite macro

Introducing the __ASM_RSB_STUFF code directly into the macro to prepare
the possibility to do both half RSB stuffing and RSB overwriting with it.

Signed-off-by: William Roche <william.roche@oracle.com>
Co-developed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Signed-off-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Reviewed-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>
(cherry picked from commit fe603bb0b80036523fb34b6c035d1ee66a26262f)

Orabug: 29660924

Signed-off-by: William Roche <william.roche@oracle.com>
Reviewed-by: Darren Kenny <darren.kenny@oracle.com>
Acked-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: Brian Maly <brian.maly@oracle.com>
Conflicts:
	arch/x86/include/asm/spec_ctrl.h

Signed-off-by: Brian Maly <brian.maly@oracle.com>