Fix an off-by-one error when populating mincore(2) entries for
skipped entries.  lastvecindex references the last valid byte,
so the new bytes should come after it.

Approved by:	re (kib)
MFC after:	1 week
- Document the UQUAD sysctl variants.
- Clarify that exactly one of the "access" flags is required and
  list the optional flags in a separate list.  Prefer bundling
  CTLFLAG_TUN into the access flag by not documenting it as an
  optional flag to set.

Approved by:	re (glebius)
MFC after:	1 week
After r255294, building lib/msun's symbol map (using clang as the
preprocessor) gives the following error:

--- Version.map ---
<stdin>:287:4: error: invalid preprocessing directive
        # Implemented as weak aliases for imprecise versions
          ^
1 error generated.

Change the comment to a C-style one, to prevent this error.

Approved by:	re (hrs)
After r255321, clang uses libc++ by default.  This leads to a lot of
errors when you enable WITH_GNUCXX to build libstdc++, since it will
include C++ headers from the libc++ installation under ${WORLDTMP}, and
those are not compatible with libstdc++ at all.

To fix this, add -stdlib=libstdc++ to CXXFLAGS when building libstdc++
(and its companion libsupc++) with clang.

Approved by:	re (delphij)