RFR: 8199138: Add RISC-V support to Zero

[John Paul Adrian Glaubitz]

Hello!

Please review this small change which adds basic support for the riscv64
target for Linux/Zero [1].

I have reduced the complexity of the patch as some of the changes from
the previous change set are not necessary, in particular the changes
to config.{guess,sub}, the definition of EM_RISCV (which is already defined
by the Linux kernel headers now provided the kernel is recent enough).

Additionally, I'm leaving the change for os::get_summary_cpu_info() out
as I would like to clean up this code a bit first - it's rather inconsistent
considering whether the arch override is used for Hotspot arches only or
also for Zero arches.

Thanks,
Adrian

> [1] http://cr.openjdk.java.net/~glaubitz/8199138/webrev.02/

-- 
 .''`.  John Paul Adrian Glaubitz
: :' :  Debian Developer - glaubitz at debian.org
`. `'   Freie Universitaet Berlin - glaubitz at physik.fu-berlin.de
  `-    GPG: 62FF 8A75 84E0 2956 9546  0006 7426 3B37 F5B5 F913