[ELF][RISCV] Assign st_shndx of __global_pointer$ to 1 if .sdata does not exist

This essentially reverts the code change of D63132 and switches to a simpler approach.

In an executable/shared object, st_shndx of a symbol can be:

1) SHN_UNDEF: undefined symbol (or canonical PLT)
2) SHN_ABS: absolute symbol
3) any other value (usually a regular section index) represents a relative symbol.
  The actual value does not matter.

Many ld.so (musl, all archs except MIPS of FreeBSD rtld-elf) even treat 2) and 3)
the same. If .sdata does not exist, it does not matter what value/section
__global_pointer$ has, as long as it is relative (otherwise there will be a pedantic
lld error. See D63132). Just set the st_shndx arbitrarily to 1.

Dummy st_shndx=1 may be used by __rela_iplt_start, linker-script-defined symbols outside a section, __dso_handle, etc.

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D66798

llvm-svn: 370172