[llvm-readobj][RISCV] Support dumping PT_RISCV_ATTRIBUTES

This patch drops the prefix `PT_RISCV_` when dumping `PT_RISCV_ATTRIBUTES`.

GNU readelf dumps it as `RISCV_ATTRIBUT`. Because GNU readelf uses
something like `%-14.14s` so only the first 14 bytes are printed.

Differential Revision: https://reviews.llvm.org/D128493