Factorize macro definition code in macrotab.c

Factorize macro definition code in macrotab.c

When first fixing splay tree key leaks in macrotab.c, some duplicated code
logic was factorized.
The key leaks will be fixed in libiberty, but the code factorization
is better kept in any case.

gdb/ChangeLog
2019-02-06  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* macrotab.c (macro_define_internal): New function that
	factorizes macro_define_object_internal and macro_define_function
	code.
	(macro_define_object_internal): Use macro_define_internal.
	(macro_define_function): Likewise.