[ELF] Make the rule to create relative relocations in a writable section stricter

The current rule is loose: `!Sym.IsPreemptible || Expr == R_GOT`.

When the symbol is non-preemptable, this allows absolute relocation
types with smaller numbers of bits, e.g. R_X86_64_{8,16,32}. They are
disallowed by ld.bfd and gold, e.g.

    ld.bfd: a.o: relocation R_X86_64_8 against `.text' can not be used when making a shared object; recompile with -fPIC

This patch:

a) Add TargetInfo::SymbolicRel to represent relocation types that resolve to a
symbol value (e.g. R_AARCH_ABS64, R_386_32, R_X86_64_64).

  As a side benefit, we currently (ab)use GotRel (R_*_GLOB_DAT) to resolve
  GOT slots that are link-time constants. Since we now use Target->SymbolRel
  to do the job, we can remove R_*_GLOB_DAT from relocateOne() for all targets.
  R_*_GLOB_DAT cannot be used as static relocation types.
b) Change the condition to `!Sym.IsPreemptible && Type != Target->SymbolicRel || Expr == R_GOT`.

Some tests are caught by the improved error checking (ld.bfd/gold also
issue errors on them). Many misuse .long where .quad should be used
instead.

Reviewed By: ruiu

Differential Revision: https://reviews.llvm.org/D63121

llvm-svn: 363059