gdb: Use std::abs instead of abs on LONGEST types

gdb: Use std::abs instead of abs on LONGEST types

Use std::abs so that we get the C++ overloaded version that matches
the argument type instead of the C abs function which is only for int
arguments.

There should be no user visible change after this commit.

gdb/ChangeLog:

	* gdbtypes.c (create_array_type_with_stride): Use std::abs not
	abs.