Reland D65242 "[ELF] More dynamic relocation packing""

This fixed a bug in r369488. When config->isRela is false, i->r_addend
is not initialized (see encodeDynamicReloc). So we should check
config->isRela before accessing r_addend:

- if (j - i < 3 || i->r_addend)
+ if (j - i < 3 || (config->isRela && i->r_addend != 0))

Original description:

Currently, with Android dynamic relocation packing, only relative
relocations are grouped together. This patch implements similar
packing for non-relative relocations.

The implementation groups non-relative relocations with the same
r_info and r_addend, if using RELA. By requiring a minimum group
size of 3, this achieves smaller relocation sections. Building Android
for an ARM32 device, I see the total size of /system/lib decrease by
392 KB.

Grouping by r_info also allows the runtime dynamic linker to implement
an 1-entry cache to reduce the number of symbol lookup required. With
such 1-entry cache implemented on Android, I'm seeing 10% to 20%
reduction in total time spent in runtime linker for several executables
that I tested.

As a simple correctness check, I've also built x86_64 Android and booted
successfully.

Differential Revision: https://reviews.llvm.org/D65242
Patch by Vic Yang

llvm-svn: 369507