Support LLVM linked ELF RISC-V executable

[metarutaiga]

Summary

I tried to use LLVM to build my executable, but it crashed always.
Then I found it did't implement some relocation.

Impact

Implement R_RISCV_BRANCH / R_RISCV_JAL / R_RISCV_RVC_JUMP / R_RISCV_RVC_BRANCH

Testing

#include <stdio.h>

int main(int argc, char const* argv[])
{
  printf("Hello world\n");
  return 0;
}

clang -Os --target=riscv32-esp-elf -march=rv32imc -mabi=ilp32 -Wl,-e,main,-r,-T,binfmt/libelf/gnu-elf.ld

[xiaoxiang781216]

@metarutaiga please squash into one patch.