gdb/testsuite: Remove hard coded addresses from expected results

In commit:

  commit 6b8c53f2
  Date:   Sat Feb 8 21:26:31 2020 +0000

      gdb/testsuite/fortran: Add mixed language stack test

The test incorrectly included two hard coded addresses in the expected
output, this commit replaces them with the $hex pattern.

gdb/testsuite/ChangeLog:

	* gdb.fortran/mixed-lang-stack.exp: Replace two hard coded address
	with $hex.