Don't put JIT_READER_DIR into help text

Don't put JIT_READER_DIR into help text

The 80-column-help-string self-test can fail if gdb's install
directory is too long, because the help for "jit-reader-load" includes
JIT_READER_DIR.

This help text is actually somewhat misleading, though.
JIT_READER_DIR is not actually used directly -- instead the relocated
variant is used.

This patch adds a new "show jit-reader-directory" command and changes
the help text to refer to this instead.  I considered adding a "set"
command as well, but since absolute paths are acceptable here, and
since this is a very niche command anyway, I figured there was no need
to bother.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32357
Reviewed-By: Kévin Le Gouguec <legouguec@adacore.com>
Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-By: Andrew Burgess <aburgess@redhat.com>