move sparc-sol-thread.c back into sol-thread.c.

The routines in sparc-sol-thread used to be SPARC-specific (and
documented as such in the ptrace man page), and therefore hosting them
in a sparc-specific file made sense.  However, newer versions of
Solaris now use those callbacks (Solaris 10 Update 10, apparently),
and thus the note about these callbacks being specific to SPARC
was removed.

So this patch deletes sparc-sol-thread.c and moves the code back
inside sol-thread.c.

gdb/ChangeLog:

        PR tdep/15420:
        * sol-thread.c (ps_lgetxregsize, ps_lgetxregs, ps_lsetxregs):
        New functions, directly copied from sparc-sol-thread.c.
        * sparc-sol-thread.c: Delete.
        * configure.ac: Remove code handling sparc-solaris-thread.c.
        * configure: Regenerate.