Reland "[JITLink] Add an initial implementation of JITLink for ELF/LoongArch"

This implementation supports basic relocation types and adds EHFrame,
Got/Plt handling passes.
This patch also enables JIT support for LoongArch64.

With this patch, I successfully run hello.ll and simple_throw.ll
(which is generated from test-suite/SingleSource/Regression/C++/EH/simple_throw.cpp)
using the `lli` command with options `--jit-kind=orc --jit-linker=jitlink`.

Note: `hasJIT` property of LoongArch32 remains false as there is no
validation environment.

New changes: Since LoongArch does not support RuntimeDyld, JITLink is set
by default.  Add a null-terminator to eh-frame sections. This should fix
the test failure on LoongArch bot.
(https://lab.llvm.org/staging/#/builders/236/builds/896)

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D141036