Fix EFI loader reproducibility with lld

[dim]

Summary

When the various loaders under stand/efi are built, the resulting
binaries differ over multiple runs, even if WITH_REPRODUCIBLE_BUILD is
used. This is caused by lld multithreading and the custom linker scripts
for the loaders, and affects the following binaries:

• loader_4th.efi
• loader_4th.sym
• loader_4th.sym.full
• loader_lua.efi
• loader_lua.sym
• loader_lua.sym.full
• loader_simp.efi
• loader_simp.sym
• loader_simp.sym.full

Work around this by disabling lld threading for these binaries.

MFC after: 3 days

[emaste]

OK as a workaround but we ought to report this upstream

[imp]

Looks good, assuming we don't already hsve a LDFLAGS.lld that we add to LDFLAGS like we do for CFLAGS.

[cperciva]

In D46271#1055178, @imp wrote:

Looks good, assuming we don't already hsve a LDFLAGS.lld that we add to LDFLAGS like we do for CFLAGS.

We do have LDFLAGS.lld, and using that works (without needing the .if/.endif).

[dim]

Use LDFLAGS.lld.

[dim]

Undo arc stupidity.