selftests/nolibc: use lld to link loongarch binaries

selftests/nolibc: use lld to link loongarch binaries

LLVM 21 switched to -mcmodel=medium for LoongArch64 compilations.
This code model uses R_LARCH_ECALL36 relocations which might not be
supported by GNU ld which to nolibc testsuite uses by default.
ld will not resolve the relocation and all function calls will end up
as busy loops.

Use lld instead.

We can not switch to lld for all LLVM builds, as it does not support all
necessary architectures.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>