debug/elf: add riscv attributes definitions

debug/elf: add riscv attributes definitions

This CL adds `riscv.attributes` related ELF section header
type and program header type according to
[RISC-V ELF Specification](https://github.com/riscv-non-isa/riscv-elf-psabi-doc/releases/download/v1.0/riscv-abi.pdf)

Also an riscv64/linux testcase binary built from:

```
gcc -march=rv64g -no-pie -o gcc-riscv64-linux-exec hello.c
strip gcc-riscv64-linux-exec
```

Fixes #72843

Change-Id: I7710a0516f69141c0efaba71dd997f05b4c88421
Reviewed-on: https://go-review.googlesource.com/c/go/+/657515
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>