Posix: Move magic DSO registration code to rt.dso druntime module

[kinke]

Addressing #3786.

[JohanEngelen]

Very nice @kinke

[kinke]

I can't reproduce the MSan failures

912: Failed Tests (2):
912:   LDC :: sanitizers/msan_noerror.d
912:   LDC :: sanitizers/msan_uninitialized.d

from the single failing LLVM 9 GH Actions job, with -DBUILD_SHARED_LIBS=ON -DRT_SUPPORT_SANITIZERS=ON - here on Ubuntu 20.04 with LLVM 12.0.0. Maybe caused by a difference in CRT ctor execution order, just a wild guess.

[JohanEngelen]

Maybe caused by a difference in CRT ctor execution order, just a wild guess.

Could it be fixed by changing link order ?

[kinke]

Doesn't look like it, putting it as first object file in the linker cmdline hasn't changed anything. Some more tests have shown that those 2 MSan tests on Ubuntu 18.04 fail with shared default libs - at least with LLVM 8 & 9 for GH Actions; LLVM 11 & 12 work with the same image (and shared libs). So I guess this has to do with the MSan version.

Further tests in #3858 with lld and bfd linkers show that this does seem to fix #3786, the LTO lit-tests with lld v12.0.1 now succeed. There's apparently one regression with bfd though, runnable/test14901.d failing for the GH Actions LLVM 11 job with BUILD_SHARED_LIBS=ON (which is green in #3848 alone). All other jobs seem unaffected, i.e., #3849 is the only other problem.

[kinke]

On Linux, all 3 linkers are CI-tested now:

• Azure & Travis: lld (from LDC-LLVM)
• GH Actions: gold
• Cirrus & Circle: bfd (incl. 1 Ubuntu 16.04 job)

[kinke]

This also fixes a general issue with LLD v13 - with current master, a hello-world fails to link (but works with LLD v10, and according to CI, with v12 too):

ld: error: undefined hidden symbol: __start___minfo
>>> referenced by hello.d
>>>               hello.o:(ldc.register_dso)

ld: error: undefined hidden symbol: __stop___minfo
>>> referenced by hello.d
>>>               hello.o:(ldc.register_dso)

Not sure why it works with this, but it does.