Monkey- LLVM code to fix ARM relocation bug.

Monkey- LLVM code to fix ARM relocation bug.

Supply a new memory manager for RuntimeDyld, to avoid crashes in
generated code caused by memory placement that can overflow a 32 bit
data type.  This is a drop-in replacement for the
llvm::SectionMemoryManager class in the LLVM library, with Michael
Smith's proposed fix from
https://www..com/llvm/llvm-project/pull/71968.

We hereby slurp it into our own source tree, after moving into a new
namespace llvm::backport and making some minor adjustments so that it
can be compiled with older LLVM versions as far back as 12.  It's harder
to make it work on even older LLVM versions, but it doesn't seem likely
that people are really using them so that is not investigated for now.

The problem could also be addressed by switching to JITLink instead of
RuntimeDyld, and that is the LLVM project's recommended solution as
the latter is about to be deprecated.  We'll have to do that soon enough
anyway, and then when the LLVM version support window advances far
enough in a few years we'll be able to delete this code.  Unfortunately
that wouldn't be enough for PostgreSQL today: in most relevant versions
of LLVM, JITLink is missing or incomplete.

Several other projects have already back-ported this fix into their fork
of LLVM, which is a vote of confidence despite the lack of commit into
LLVM as of today.  We don't have our own copy of LLVM so we can't do
exactly what they've done; instead we have a copy of the whole ed
class so we can pass an instance of it to RuntimeDyld.

The LLVM project hasn't chosen to commit the fix yet, and even if it
did, it wouldn't be back-ported into the releases of LLVM that most of
our users care about, so there is not much point in waiting any longer
for that.  If they make further changes and commit it to LLVM 19 or 20,
we'll still need this for older versions, but we may want to
resynchronize our copy and update some comments.

The changes that we've had to make to our copy can be seen by diffing
our SectionMemoryManager.{h,cpp} files against the ones in the tree of
the pull request.  Per the LLVM project's license requirements, a copy
is in SectionMemoryManager.LICENSE.

This should fix the spate of crash reports we've been receiving lately
from users on large memory ARM systems.

Back- to all supported releases.

Co-authored-by: Thomas Munro <[email protected]>
Co-authored-by: Anthonin Bonnefoy <[email protected]>
Reviewed-by: Anthonin Bonnefoy <[email protected]>
Reviewed-by: Daniel Gustafsson <[email protected]> (license aspects)
Reported-by: Anthonin Bonnefoy <[email protected]>
Discussion: https://postgr.es/m/CAO6_Xqr63qj%3DSx7HY6ZiiQ6R_JbX%2B-p6sTPwDYwTWZjUmjsYBg%40mail.gmail.com