x86/entry: Move PUSH_AND_CLEAR_REGS out of error_entry()

The macro idtentry() (through idtentry_body()) calls error_entry()
unconditionally even on XENPV. But XENPV needs to only push and clear
regs.

PUSH_AND_CLEAR_REGS in error_entry() makes the stack not return to its
original place when the function returns, which means it is not possible
to convert it to a C function.

Carve out PUSH_AND_CLEAR_REGS out of error_entry() and into a separate
function and call it before error_entry() in order to avoid calling
error_entry() on XENPV.

It will also allow for error_entry() to be converted to C code that can
use inlined sync_regs() and save a function call.

  [ bp: Massage commit message. ]

Signed-off-by: Lai Jiangshan <jiangshan.ljs@antgroup.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20220503032107.680190-4-jiangshanlai@gmail.com