target/xtensa linux-user support.

- small cleanup for xtensa registers dumping (-d cpu);
- add support for debugging linux-user process with xtensa-linux-gdb
  (as opposed to xtensa-elf-gdb), which can only access unprivileged
  registers;
- enable MTTCG for target/xtensa;
- cleanup in linux-user/mmap area making sure that it works correctly
  with limited 30-bit-wide user address space;
- import xtensa-specific definitions from the linux kernel,
  conditionalize user-only/softmmu-only code and add handlers for
  signals, exceptions, process/thread creation and core registers dumping.

# gpg: Signature made Fri 16 Mar 2018 16:46:19 GMT
# gpg:                using RSA key 51F9CC91F83FA044
# gpg: Good signature from "Max Filippov <filippov@cadence.com>"
# gpg:                 aka "Max Filippov <max.filippov@cogentembedded.com>"
# gpg:                 aka "Max Filippov <jcmvbkbc@gmail.com>"
# Primary key fingerprint: 2B67 854B 98E5 327D CDEB  17D8 51F9 CC91 F83F A044

* remotes/xtensa/tags/20180316-xtensa:
  MAINTAINERS: fix W: address for xtensa
  qemu-binfmt-conf.sh: add qemu-xtensa
  target/xtensa: add linux-user support
  linux-user: drop unused target_msync function
  linux-user: fix target_mprotect/target_munmap error return values
  linux-user: fix assertion in shmdt
  linux-user: fix mmap/munmap/mprotect/mremap/shmat
  target/xtensa: support MTTCG
  target/xtensa: use correct number of registers in gdbstub
  target/xtensa: mark register windows in the dump
  target/xtensa: dump correct physical registers

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

# Conflicts:
#	linux-user/syscall.c