[ELF] Change sh_link of .rel{,a}.plt to make GNU strip happy

Summary:
D52830 sets sh_link to .symtab in static link, which breaks executable stripped by GNU strip.
It may also be odd that .rela.plt (SHF_ALLOC) points to .symtab (non-SHF_ALLOC).

Change the logic on pcc's suggestion.

Before:

% clang -fuse-ld=lld -static -xc =(printf 'int main(){}') # or gcc
% strip a.out; ./a.out
unexpected reloc type in static binary[1]    61634 segmentation fault  ./a.out

Reviewers: ruiu, grimar, emaste, espindola

Reviewed By: ruiu

Subscribers: pcc, arichardson, llvm-commits

Differential Revision: https://reviews.llvm.org/D53993

llvm-svn: 345899