[binutils-gdb] Return std::string from gdb_bfd_errmsg

[sergiodj+buildbot at sergiodj dot net]

*** TEST RESULTS FOR COMMIT 803c08d083556154cd4d27368e58b359e9de8b93 ***

Author: Tom Tromey <tom@tromey.com>
Branch: master
Commit: 803c08d083556154cd4d27368e58b359e9de8b93

Return std::string from gdb_bfd_errmsg

This changes gdb_bfd_errmsg to return a std::string, removing a
cleanup.  This approach may be slightly less efficient than the
previous code, but I don't believe this is very important in this
situation.

gdb/ChangeLog
2018-09-13  Tom Tromey  <tom@tromey.com>

	* utils.h (gdb_bfd_errmsg): Return std::string.
	* exec.c (exec_file_attach): Update.
	* compile/compile-object-load.c (compile_object_load): Update.
	* utils.c (gdb_bfd_errmsg): Return std::string.