Drop building linux-user targets on HPPA or m68k host systems
and add safe_syscall support for i386, aarch64, arm, ppc64 and
s390x.

# gpg: Signature made Tue 28 Jun 2016 19:31:16 BST
# gpg:                using RSA key 0xB44890DEDE3C9BC0
# gpg: Good signature from "Riku Voipio <riku.voipio@iki.fi>"
# gpg:                 aka "Riku Voipio <riku.voipio@linaro.org>"
# Primary key fingerprint: FF82 03C8 C391 98AE 0581  41EF B448 90DE DE3C 9BC0

* remotes/riku/tags/pull-linux-user-20160628: (24 commits)
  linux-user: Provide safe_syscall for ppc64
  linux-user: Provide safe_syscall for s390x
  linux-user: Provide safe_syscall for aarch64
  linux-user: Provide safe_syscall for arm
  linux-user: Provide safe_syscall for i386
  linux-user: fix x86_64 safe_syscall
  linux-user: don't swap NLMSG_DATA() fields
  linux-user: fd_trans_host_to_target_data() must process only received data
  linux-user: add missing return in netlink switch statement
  linux-user: update get_thread_area/set_thread_area strace
  linux-user: fix clone() strace
  linux-user: add socket() strace
  linux-user: add socketcall() strace
  linux-user: Support F_GETPIPE_SZ and F_SETPIPE_SZ fcntls
  linux-user: Fix wrong type used for argument to rt_sigqueueinfo
  linux-user: Create a hostdep.h for each host architecture
  user-exec: Remove unused code for OSX hosts
  user-exec: Delete now-unused hppa and m68k cpu_signal_handler() code
  configure: Don't allow user-only targets for unknown CPU architectures
  configure: Don't override ARCH=unknown if enabling TCI
  ...

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>