RISC-V: Fix the VDSO symbol generaton for binutils-2.35+

We were relying on GNU ld's ability to re-link executable files in order
to extract our VDSO symbols.  This behavior was deemed a bug as of
binutils-2.35 (specifically the binutils-gdb commit a87e1817a4 ("Have
the linker fail if any attempt to link in an executable is made."), but
as that has been backported to at least Debian's binutils-2.34 in may
manifest in other places.

The previous version of this was a bit of a mess: we were linking a
static executable version of the VDSO, containing only a subset of the
input symbols, which we then linked into the kernel.  This worked, but
certainly wasn't a supported path through the toolchain.  Instead this
new version parses the textual output of nm to produce a symbol table.
Both rely on near-zero addresses being linkable, but as we rely on weak
undefined symbols being linkable elsewhere I don't view this as a major
issue.

Fixes: e2c0cdfb ("RISC-V: User-facing API")
Signed-off-by: Palmer Dabbelt <p...