linux-user: Support tilegx architecture in linux-user

linux-user: Support tilegx architecture in linux-user

Add main working flow feature, system call processing feature, and elf64
tilegx binary loading feature, based on Linux kernel tilegx 64-bit
implementation.

[rth: Moved all of the implementation of atomic instructions to a later patch.]

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <BLU436-SMTP938552D42808AA60634582B9660@phx.gbl>
Signed-off-by: Richard Henderson <rth@twiddle.net>