Split the switch_to() wrapper out of asm-generic/system.h

Split the switch_to() wrapper out of asm-generic/system.h into its own
asm-generic/system.h as part of the asm/system.h disintegration.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>