Enable LLDB by default on platforms where we build Clang

[emaste]

Test Plan

Check that lldb is built if no WITH_ or WITHOUT_ in src.conf

[emaste]

Rebase and enable for arm64 as well

[jhb]

Will be nice to get more platforms on the default yes list. :)

[imp]

share/mk/src.opts.mk

249

I'dd add LLDB to DEFAULT_YES options unconditionally.
And I'd change this .if to to be the contrapositive of the one on line 241.

[emaste]

share/mk/src.opts.mk

249

I think we can instead enable LLDB by default on the same platforms that build Clang by default (i.e., i386, amd64, arm*, powerpc*)

[emaste]

On 32- and 64-bit x86, arm, and powerpc we build Clang by default. Build LLDB by default on these as well.

[emaste]

For reference, times from a few test builds:

make -j16                                          buildworld  12448.13s user 2164.26s system 1249% cpu 19:29.06 total
make -j16 WITH_DEBUG_FILES=yes                     buildworld  12860.29s user 2484.72s system 1285% cpu 19:53.55 total
make -j16                         WITH_LLDB=yes    buildworld  13638.69s user 2472.67s system 1296% cpu 20:42.47 total
make -j16 WITH_DEBUG_FILES=yes    WITH_LLDB=yes    buildworld  13992.22s user 2526.42s system 1306% cpu 21:04.76 total
make -j16 WITHOUT_DEBUG_FILES=yes WITHOUT_LLDB=yes buildworld  12557.10s user 2479.58s system 1260% cpu 19:53.12 total

[emaste]

For now just explicitly default LLDB to YES on arm/arm64/amd64, and NO elsewhere.

I would like to build it in those cases where clang is /usr/bin/cc, but it's not yet usable for live debugging on i386 so there's no point in enabling it there yet. We could use BROKEN_OPTIONS to disable it on i386, but that's not quite right: it will build on i386, and can debug core files from other architectures or connect to remote targets.

[emaste]

First pass added in rS292350
https://svnweb.freebsd.org/changeset/base/292350
I did it only on amd64 and arm64 for now.