RISC-V: Define and use GOT entry size constants for TLS.

As the size calculation is split by global and local symbols, using a
shared constant definition for its size improves clarity.

bfd/
    * elfnn-riscv.c: Add macros for sizes of a normal GOT entry, TLS GD and
    TLS IE entry.
    (allocate_dynrelocs): Replace GOT size expressions with the new
    constants.
    (riscv_elf_size_dynamic_sections): Likewise.
    (riscv_elf_relocate_section): Likewise.