src.opts.mk: enable LLVM_UNWIND by default for all archs

Only sparc64 did not enable LLVM_LIBUNWIND. After r356513 LLVM_LIBUNWIND
should at least build on sparc64. The old DWARF unwinder will be removed
along with GCC 4.2.1 in the near future, so switch sparc64 to use LLVM's
unwinder in advance of the removal. Someone with access to the obsolete
sparc64 hardware supported by FreeBSD will have to test, and investigate
any failures. I will gladly help, but I don't have any suitable hardware
myself.

PR: 233405