src.opts.mk: switch LLVM toolchain to opt-out

After PowerPC switched in r356111, the list of targets using LLVM as the
default toolchain is much longer than those not using it. Switch the
sense of the test to exclude those not using LLVM.

Targets not using LLVM is currently mips, riscv5, and sparc64; work is
in progress to migrate the first two to LLVM.