riscv: Change the relocations handled for RTLD_BOOTSTRAP

The RTLD_BOOTSTRAP branch is used to relocate ld.so itself.  It only
needs to handle RELATIVE, GLOB_DAT, and the symbolic relocation type
(R_RISCV_{32,64}).  NONE and IRELATIVE can be removed.

The code relies on ld.so having DT_RELACOUNT so that the RTLD_BOOTSTRAP
branch does not need handle RELATIVE.  Drop this minor size
optimization for clarity.

Acked-by: Palmer Dabbelt <palmer@rivosinc.com>