Fix issue 21089: spawn linker with vfork to avoid need to overcommit memory

[FeepingCreature]

DMD is large, ld is small.

Replaces #11476 as it is simpler and offers better error reporting.

vfork is removed from POSIX-2008, but it's not like it'll ever actually be removed from implementations, since it's easy to provide and other nondeprecated functions still use the mechanism.

[dlang-bot]

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21089	minor	With vm.overcommit_memory=0, DMD can't link if it uses more than half the total (ram+swap) memory in the system.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#11497"