debug/elf: add R_RISCV_32_PCREL relocation

This were missed in CL 107339 as it is not documented (yet) in
https://github.com/riscv/riscv-elf-psabi-doc/blob/master/riscv-elf.md

But binutils already uses it. See
https://github.com/riscv/riscv-elf-psabi-doc/issues/36

Change-Id: I1b084cbf70eb6ac966136bed1bb654883a97b6a9
Reviewed-on: https://go-review.googlesource.com/134015


Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>