[llvm] Fix LLVM JIT when used with multiple AppDomains.

[monojenkins]

!! This PR is a copy of mono/mono#18824, please do not edit or review it in this repo !!
!! Merge the PR only after the original PR is merged !!

---

We currently reuse LLVM's command line parsing system to configure and
enable passes. When using the default code generator pass configuration,
the ImplicitNullChecks and X86CallFrameOptimization passes can only
be enabled or disabled via the command line argument parser: the
cl::opts controlling these (and many other) passes are private to
their translation units, and TargetPassConfig/X86PassConfig read
from these cl::opts with no other means to override these values.

Unfortunately, some LLVM command line options have restrictions on the
number of times they may be set, and running the LLVM command line
argument parser more than once can fail, because the 'number of
occurrences' counter is stored in each global cl::opt. This causes
several tests to fail--one such test is unhandled-exception-7.cs.

This change:

• removes the lazy LLVM JIT initialization logic and instead
  runs this initialization once during mini_init,

• moves some stray JIT-only code from mini-llvm.c to llvm-jit.cpp,

• wraps the declarations in domain-internals.h with a
  G_BEGIN_DECLS/G_END_DECLS pair, so that they avoid name
  mangling when included in C++ source, and

• removes exception_cb and emitted_cb: neither were actually used
  by live code.