gdb.base/nodebug.exp: Rename called functions

gdb.base/nodebug.exp: Rename called functions

I'm seeing these failures on my system:

  FAIL: gdb.base/nodebug.exp: p (double) mult (2.0, 3.0)
  FAIL: gdb.base/nodebug.exp: p ((double (*) (double, double)) mult)(2.0f, 3.0f)
  FAIL: gdb.base/nodebug.exp: p ((double (*) (double, double)) mult)(2, 3)

The problem is simply that GDB is finding a symbol named "mult" from
glibc's debug info:

  (gdb) ptype mult
  type = enum expression_operator {var, num, lnot, mult, divide, module, plus, minus, less_than, greater_than, less_or_equal, greater_or_equal, equal, not_equal, land, lor,  qmop}

  (gdb) info types expression_operator
  All types matching regular expression "expression_operator":

  File plural-exp.h:
  enum expression_operator;

Fix this by unloading symbols from shared libraries.

gdb/testsuite/ChangeLog:
2017-09-14  Pedro Alves  <palves@redhat.com>

	* gdb.base/nodebug.exp (nodebug_runto): New procedure.
	(top level): Use it instead of runto.