Adding cross-compilation and static support to embedded lld for linux.

[redvers]

Changes required to support cross-compilation and static support.

[redvers]

Just making notes as I validate and explore:

0. Appropriate distinguishing of -L and -rpath. Ensure that there are no -rpaths when -static.
1. What about arguments that the previous execution to clang had that don't make sense for lld? For example, -mcx16?
2. There is a divergence between the arguments that we have provided and those that clang generates internally.

Differences on my local workstation:

WIP implementation	clang generation
	-z relro
--hash-style=both	--hash-style=gnu
	--build-id

3. --as-needed and --no-as-needed is implemented by clang but not us. Why? Does it matter?

4. The logic that clang uses to locate the library paths and correct flags for lld to use for the various cases we have to deal with is located in clang/lib/Driver/ToolChains. It appears to "guess" the paths for things like the candidate GCC libraries in much the same way we do:

Ubuntu clang version 18.1.3 (1ubuntu1)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/13
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/13
Candidate multilib: .;@m64
Selected multilib: .;@m64

It may be a good idea to at least initially synchronize our search paths to the ones that they use for "least surprise".

5. Adding ZLIB support to LLVM (required for x86-musl), seems to break all the things.
6. arm64 Apple Darwin image does not seem to have zlib installed.

[redvers]

The -fsanitize=address,undefined operation adds the following to the linker:

--whole-archive
/usr/lib/llvm-18/lib/clang/18/lib/linux/libclang_rt.asan_static-x86_64.a
--no-whole-archive
--whole-archive
/usr/lib/llvm-18/lib/clang/18/lib/linux/libclang_rt.asan-x86_64.a
--no-whole-archive
--dynamic-list=/usr/lib/llvm-18/lib/clang/18/lib/linux/libclang_rt.asan-x86_64.a.syms

Since it requires clang to be installed, this may not be worth the squeeze.

[redvers]

Going to merge this into the other PR:

TODO:
0. sanitize.

1. Inventory and implement LLVM / Cross compilation environmental variables.
2. Refactor and doc.
3. Investigate how cross compilation works outside of CI to better understand.
4. Consider ponyc flag to force to use legacy linker.