Fix bootstrapping to actually build lldb-tblgen for later use

Because MK_LLDB=no is in BSARGS, the bootstrap-tools recursive make does
not add lldb-tblgen to _clang_tblgen, causing it to not be built. This
means that the build currently always uses the host's lldb-tblgen
(which, whilst currently it appears to work, could in future break if
TableGen backends are added or altered) and, if it doesn't exist (either
because the current FreeBSD system was built with it disabled, or you're
building on macOS/Linux), fails. Linux and macOS cross-builds used to
work simply because LLDB was previously in BROKEN_OPTIONS when building
on non-FreeBSD.

Instead, move MK_LLDB=no from BSARGS to XMAKE. This ensures that the
lib/clang build in cross-tools continues to not build LLDB parts for the
bootstrap toolchain (both to save time/space on FreeBSD, and because our
vendored LLDB does not include the macOS and Linux host files so those
would fail to build).

The DIRDEPS target is updated to move MK_LLDB=no from the BSARGS block
that mirrors Makefile.inc1 to the line that disables additional
toolchain components. The DIRDEPS build likely suffers from the same
issue currently, but having never used it and not being familiar with
how it works I am leaving that as-is. If it does suffer from the same
issue it should be easily reproducible by renaming /usr/bin/lldb-tblgen
or moving it to a directory not in PATH.

Fixes: 31ba4ce8898f ("Allow bootstrapping llvm-tblgen on macOS and Linux")
MFC after: 1 week
Reviewed by: dim, emaste, imp
Differential Revision: https://reviews.freebsd.org/D31531

(cherry picked from commit 1e4c802913af619ac15741bbd276e1141ca17dc9)