riscv: Remove support for variable page sizes

_dl_var_init is used to patch the read-only data section after
relocation.  Several architectures use this to update
GLRO(page_size) with the correct value for the static dlopen case,
where _rtld_global_ro has not been initialized by the dynamic
loader.

RISC-V does not need this.  The RISC-V Instruction Set Manual,
Volume II: Privileged Architecture, Document Version
20190608-Priv-MSU-Ratified says this:

    After much deliberation, we have settled on a conventional
    page size of 4 KiB for both RV32 and RV64. We expect this
    decision to ease the porting of low-level runtime software
    and device drivers. The TLB reach problem is ameliorated by
    transparent superpage support in modern operating systems
    [2]. Additionally, multi-level TLB hierarchies are quite
    inexpensive relative to the multi-level cache hierarchies
    whose address space they map.

    [2] Juan Navarro, Sitaram Iyer, Peter Druschel, and
      Alan Cox. Practical, transparent operating system support
      for superpages.  SIGOPS Oper. Syst. Rev., 36(SI):89–104,
      December 2002.

This means that the initialization of
_rtld_global_ro._dl_page_size in elf/rtld.c with EXEC_PAGESIZE
is sufficient for RISC-V.

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