[ELF][PPC64] Support long branch thunks with addends

[ELF][PPC64] Support long branch thunks with addends

Fixes PPC64 part of PR40438

  // clang -target ppc64le -c a.cc
  // .text.unlikely may be placed in a separate output section (via -z keep-text-section-prefix)
  // The distance between bar in .text.unlikely and foo in .text may be larger than 32MiB.
  static void foo() {}
  __attribute__((section(".text.unlikely"))) static int bar() { foo(); return 0; }
  __attribute__((used)) static int dummy = bar();

This patch makes such thunks with addends work for PPC64.

AArch64: .text -> `__AArch64ADRPThunk_ (adrp x16, ...; add x16, x16, ...; br x16)` -> target
PPC64: .text -> `__long_branch_ (addis 12, 2, ...; ld 12, ...(12); mtctr 12; bctr)` -> target

AArch64 can leverage ADRP to jump to the target directly, but PPC64
needs to load an address from .branch_lt . Before Power ISA v3.0, the
PC-relative ADDPCIS was not available. .branch_lt was invented to work
around the limitation.

Symbol::ppc64BranchltIndex is replaced by
PPC64LongBranchTargetSection::entry_index which take addends into
consideration.

The tests are rewritten: ppc64-long-branch.s tests -no-pie and
ppc64-long-branch-pi.s tests -pie and -shared.

Reviewed By: sfertile

Differential Revision: https://reviews.llvm.org/D70937