(subdirs, clean, distclean): Use it.
* gdb.arch/Makefile.in: Clean up Makefile. Remove rules for
regenerating Makefile, since it is generated from the top level.
* gdb.asm/Makefile.in: Likewise.
* gdb.base/Makefile.in: Likewise.
* gdb.c++/Makefile.in: Likewise.
* gdb.disasm/Makefile.in: Likewise.
* gdb.java/Makefile.in: Likewise.
* gdb.mi/Makefile.in: Likewise.
* gdb.threads/Makefile.in: Likewise.
* gdb.trace/Makefile.in: Likewise.
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