1.0.32.29: Add build flag :sb-xref-for-internals.

1.0.32.29: Add build flag :sb-xref-for-internals.

Enabling :sb-xref-for-internals in customize-target-features.lisp,
will make Sbcl collect Xref data about itself during the build. This
increases the core size drastically by about 5-6mb, but it's useful
for SBCL developers because they can now use M-? (slime-edit-uses) to
get a list of call/expansion/reference sites for internal stuff.

It may be interesting to Lisp advocacy who can now show off with
finding the use sites of standardized functions like CONS, etc. :-)

Additionally -- regardless of :sb-xref-for-internals --, we now also
collect xref data for keywords because they're "fine" names for
functions and macros, and I know of people who use MACROLET on
keywords for their DSLs.