RISC-V support

[Jim Wilson <jimw@sifive.com>]

I'm looking at the RISC-V elfutils support to help the Debian folks.
I see four testcases failing, same as Kurt Roeckx reported about 6
weeks ago.  I'm testing on a Fedora Core 29 system.

I found a trivial bug in backends/riscv_corenote.c.  It has ".offset =
1" but this is a byte offset not a register offset, so it needs to be
".offset = 8" instead.  I also added in the missing PC support.  These
two fixes then require a fix for tests/run-readelf-mixed-corenote.sh
because the eu-readelf output is now more correct than before.

There is also a missing backends/riscv_retval.c file.  I have an
initial implementation for this, but I haven't implemented the support
for structures with one or two float fields yet, as this gets a little
complicated.

With these patches, I now see two failures.  One is a glibc bug that
Andreas Schwab already fixed, where _start fails to terminate the
unwind chain.  I just don't have this patch on my system.  The other
failure is the same glibc bug in __thread_start, which apparently
isn't fixed yet, and needs the same fix Andreas already added to
_start.  So with the appropriate glibc fixes, the elfutils testsuite
should run without error on a riscv64/lp64d system using the patches I
have.

There is a problem here though.  The riscv support was written to try
to handle both 32-bit and 64-bit targets with a single elfutils
backend.  But I have 6 ABIs I need to (theoretically) handle in
riscv_retval.c.  The return_value_location function doesn't take any
ebl or elf pointer, so I can't handle it there.  I can handle it in
riscv_init.c by checking ebl and elf pointers there, and calling an
appropriate function, but I'm not sure if that is OK.  Currently, none
of the *_init.c files are using the elf pointer argument.

I noticed another problem which is that riscv_corenote.c is only
correct for riscv64, because it assumes that registers are 64-bits.
But I see that sparc has a solution for that, so I will have to take a
closer look at that and see if I can make it work for riscv.

I unfortunately can't test the 32-bit riscv support.  We don't have
working upstream support for 32-bit linux yet.  I can only test the
64-bit LP64D riscv support.

I haven't contributed to elfutils before.  So I'm looking for advice
on how to proceed.  I can send out my work in progress patches if that
is useful.  I probably should try to chop them up a bit first. I think
I have 3 parts at the moment.  One part should be OK, and one part
needs more work to be complete (but maybe incomplete is OK?), and one
part I haven't written yet.

Jim