src.opts.mk: Enable LLDB for 32-bit arm

[emaste]

Summary

It was previously disabled when we built for older 32-bit arm targets
which lack 64-bit atomics.  As we now support armv7 at a minimum there
is no need to disable LLDB.

[emaste]

riscv failure for reference:

ld: error: undefined reference: lldb_private::process_freebsd::NativeRegisterContextFreeBSD::CreateHostNativeRegisterContextFreeBSD(lldb_private::ArchSpec const&, lldb_private::process_freebsd::NativeThreadFreeBSD&)
>>> referenced by /scratch/tmp/emaste/obj/home/emaste/src/riscv.riscv64/lib/clang/liblldb/libprivatelldb.so (disallowed by --no-allow-shlib-undefined)