linux-user/uname: Return correct uname string for x86_64

linux-user/uname: Return correct uname string for x86_64

We were returning the incorrect uname string (with a hyphen, not
an underscore) for x86_64. Fix this by removing the x86_64 special
case, since the default "just use UNAME_MACHINE" behaviour suffices.
This leaves cpu_to_uname_machine() special cases for only those
architectures which need to vary the string based on runtime CPU
features.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>