x86/entry: Move SYSENTER's regs->sp and regs->flags fixups into C

The SYSENTER asm (32-bit and compat) contains fixups for regs->sp and
regs->flags.  Move the fixups into C and fix some comments while at it.

This is a valid cleanup all by itself, and it also simplifies the
subsequent patch that will fix Xen PV SYSENTER.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/fe62bef67eda7fac75b8f3dbafccf571dc4ece6b.1593191971.git.luto@kernel.org