Use debug_prefixed_printf_cond_nofunc in machoread

This changes machoread.c to use debug_prefixed_printf_cond_nofunc.  As
a side effect, the logs are now written to gdb_stdlog.  This is part
of PR gdb/7233.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=7233