linux-user: adapt uname machine to emulated CPU

This patch for linux-user adapts the output of the emulated uname()
syscall to match the configured CPU.  Tested with x86, x86-64 and arm
emulation.

Signed-off-by: Riku Voipio <riku.voipio@iki.fi>
Signed-off-by: Loïc Minier <lool@dooz.org>