Make gdb.python/python.exp more robust

Make gdb.python/python.exp more robust

I noticed gdb.python/python.exp failing on aarch64-elf like so:

FAIL: gdb.python/python.exp: Test decode_line func1 line number

This particular test expects the line number for func1 to be 19, hardcoded.

In my aarch64-elf tests gdb thinks func1 is at line 20, making the test fail.

The following patch addresses this by reading the line number information from
GDB and comparing it against the python decoded symtab information.

gdb/testsuite/ChangeLog:

2017-01-20  Luis Machado  <lgustavo@codesourcery.com>

	* gdb.python/python.exp: Check line number against what GDB thinks
	the line number is for func1.