Fix gdbserver build failure on arm-android.

The following patch...

| proc-service, extern "C"
|
| libthread_db.so calls symbols in the client (GDB), through the
| proc-service interface.  These routines must have extern "C" linkage
| so their symbol names are not mangled when GDB is built as a C++
| program.  On the GDBserver side, we were missing fallback declarations for
| all these symbols.
|
| gdb/ChangeLog:
|
|     * gdb_proc_service.h: Wrap with EXTERN_C_PUSH/EXTERN_C_POP.
|
| gdb/gdbserver/ChangeLog:
| 2015-02-27  Pedro Alves  <palves@redhat.com>
|
|     * gdb_proc_service.h: Wrap with EXTERN_C_PUSH/EXTERN_C_POP.
|     [!HAVE_PROC_SERVICE_H] (struct ps_prochandle): Forward declare.
|     [!HAVE_PROC_SERVICE_H] (ps_pdread, ps_pdwrite, ps_ptread)
|     ps_ptwrite, ps_lgetregs, ps_lsetregs, ps_lgetfpregs)
|     (ps_lsetfpregs, ps_getpid)
|     (ps_get_thread_area, ps_pglobal_lookup, ps_pstop, ps_pcontinue)
|     (ps_lstop, ps_lcontinue, ps_lgetxregsize, ps_lgetxregs)
|     (ps_lsetxregs, ps_plog): Declare.

... added a number of declarations which do not compile when cross-
compiling GDBserver on arm-android. The problem comes from type
prfpregset_t not being declared:

    /[...]/gdbserver/gdb_proc_service.h:98:47:
    error: unknown type name 'prfpregset_t'

After searching through the includes of the install we have,
I could not find that type being declared anywhere. So I did
the same as for prgregset_t, and created the typedef if the
type isn't declared.

gdb/gdbserver/ChangeLog:

        * configure.ac: Add prfpregset_t BFD_HAVE_SYS_PROCFS_TYPE check.
        * configure, config.in: Regenerate.
        * gdb_proc_service.h [HAVE_PRFPREGSET_T] (prfpregset_t):
        Declare typedef.

gdb/gdbserver/gdb_proc_service.h

@@ -71,6 +71,10 @@ typedef void *psaddr_t;
 typedef elf_gregset_t prgregset_t;
 #endif
 
+#ifndef HAVE_PRFPREGSET_T
+typedef elf_fpregset_t prfpregset_t;
+#endif
+
 /* This type is opaque in this interface.  It's defined by the user of
    libthread_db.  GDB's version is defined below.  */
 struct ps_prochandle;