Fix EFI loader reproducibility with lld

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.

Reviewed by: emaste
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D46271

(cherry picked from commit f5ce3f4ef562ea9fc4d8f9c13c268f48a5bacba7)