architecture: Add support for the RISC-V architecture. (#4305)

RISC-V is an open source ISA in development since 2010 at UCB.
For more information, see https://riscv.org/

I am adding RISC-V support to Fedora:
https://fedoraproject.org/wiki/Architectures/RISC-V

There are three major variants of the architecture (32-, 64- and
128-bit).  The 128-bit variant is a paper exercise, but the other
two really exist in silicon.  RISC-V is always little endian.

On Linux, the default kernel uname(2) can return "riscv" for all
variants.  However a patch was added recently which makes the kernel
return one of "riscv32" or "riscv64" (or in future "riscv128").  So
systemd should be prepared to handle any of "riscv", "riscv32" or
"riscv64" (in future, "riscv128" but that is not included in the
current patch).  If the kernel returns "riscv" then you need to use
the pointer size in order to know the real variant.

The Fedora/RISC-V kernel only ever returns "riscv64" since we're
only doing Fedora for 64 bit at the moment, and we've patched the
kernel so it doesn't return "riscv".

As well as the major bitsize variants, there are also architecture
extensions.  However I'm trying to ensure that uname(2) does *not*
return any other information about those in utsname.machine, so that
we don't end up with "riscv64abcde" nonsense.  Instead those
extensions will be exposed in /proc/cpuinfo similar to how flags
work in x86.

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