Merge remote-tracking branch 'remotes/rth/tags/pull-tcg-20181226' into staging

Host support for riscv64.
Dead code elimination pass.
Register allocation improvements.

# gpg: Signature made Tue 25 Dec 2018 20:52:34 GMT
# gpg:                using RSA key 64DF38E8AF7E215F
# gpg: Good signature from "Richard Henderson <richard.henderson@linaro.org>"
# Primary key fingerprint: 7A48 1E78 868B 4DB6 A85A  05C0 64DF 38E8 AF7E 215F

* remotes/rth/tags/pull-tcg-20181226: (42 commits)
  tcg: Improve call argument loading
  tcg: Record register preferences during liveness
  tcg: Add TCG_OPF_BB_EXIT
  tcg: Split out more subroutines from liveness_pass_1
  tcg: Rename and adjust liveness_pass_1 helpers
  tcg: Reindent parts of liveness_pass_1
  tcg: Dump register preference info with liveness
  tcg: Improve register allocation for matching constraints
  tcg: Add output_pref to TCGOp
  tcg: Add preferred_reg argument to tcg_reg_alloc_do_movi
  tcg: Add preferred_reg argument to temp_sync
  tcg: Add preferred_reg argument to temp_load
  tcg: Add preferred_reg argument to tcg_reg_alloc
  tcg: Add reachable_code_pass
  tcg: Reference count labels
  tcg: Add TCG_CALL_NO_RETURN
  tcg: Renumber TCG_CALL_* flags
  linux-user: Add safe_syscall for riscv64 host
  disas/microblaze: Remove unused REG_SP macro
  configure: Add support for building RISC-V host
  ...

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