Rebuild libjulia+jll 1.5 with fixed libunwind linkage

[fingolfin]

The ultimate goal of this PR is to resolve the linker issue reported here based on the pointers by @benlorenz.

For now I just enabled LibUnwind_jll as a dependency to see what goes wrong, if anything. This can't be the final version, though, as we must also make sure not to break Julia 1.3 and 1.4.

[benlorenz]

More pointers:
LIBOSXUNWIND is not defined in the build output, since there is no code in Make.inc to set that variable for the USE_SYSTEM_LIBUNWIND case on Darwin. I am not really sure why that case is not covered at all there, but I would think that this variable should be set here.
A fix for that rec_backtrace_ctx error is included in 1.5.3.
Have a look at what I posted in an earlier libjulia pull request here: #1951 (comment)

[benlorenz]

Using the Dependency pulls in LibOSXUnwind_jll version 0.0.5 which doesn't contain a proper shared library (just some stub). The build system would use 0.0.6+0 instead which contains a valid dylib.
The reason is probably that the update from 0.0.6+0 to 0.0.6+1 added a julia compat restriction to 1.6 for all 0.0.6 versions, see #2038 and JuliaRegistries/General#24077

[fingolfin]

sigh. of course.

Anyway, I actually tried to just disable use of libunwind by setting DISABLE_LIBUNWIND, based on the rationale that we just want to use the produced libjulia.dylib during build time, but never during runtime, so there should be no need for unwinding. That resulted in a ton of build failures, though, which is why I abandoned it late last night. I'll give it another try now, locally...

[benlorenz]

Linking it statically instead seems to work here, i.e.:

        LIBUNWIND:=${libdir}/libosxunwind.a

(At least for the build, I have not tested the resulting library)

[fingolfin]

Yeah, and disabling use of libunwind also works locally here... but not (yet?) in the BB file. I am playing with that right now, though

[fingolfin]

OK, using DISABLE_LIBUNWIND=1 simply is broken in the Julia build system, at least on macOS and in Julia 1.5 and master: it results in compilation errors:

./signals-mach.c:293:51: error: expected expression
        jl_critical_error(SIGSEGV, (unw_context_t*)&state,
                                                  ^
./signals-mach.c:293:37: error: use of undeclared identifier 'unw_context_t'
        jl_critical_error(SIGSEGV, (unw_context_t*)&state,
                                    ^
./signals-mach.c:503:13: error: use of undeclared identifier 'unw_context_t'
            unw_context_t *uc;
            ^
...

But the static compilation trick suggested by @benlorenz seems to work, as in: the build completes and the resulting libjulia.dylib has no unresolved references to unwind related symbols. But whether it works in the sense of "the resulting binary resolve the issues in PR #2158" remains to be seen.

Testing this w/o just merging this PR is a bit of a pain, though; gotta build the JLL locally, then build the next JLL (see #2158) using the first one. Perhaps we can simply merge this PR (it shouldn't make anything worse, after all)?

[benlorenz]

Do we need to verify that 1.3 and 1.4 still build with this change?

[fingolfin]

Pushing Julia 1.3 and 1.4 here now to see if they still build

[fingolfin]

Seems they all still build just fine. I've removed that HACk commit again.

So yeah, this could be merged... @giordano ?