linux-user: Move cpu_clone_regs() and cpu_set_tls() into linux-user

The functions cpu_clone_regs() and cpu_set_tls() are not purely CPU
related -- they are specific to the TLS ABI for a a particular OS.
Move them into the linux-user/ tree where they belong.

target-lm32 had entirely unused implementations, since it has no
linux-user target; just drop them.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Andreas Färber <afaerber@suse.de>