clean after distclean gobbles source files

[Steven Sistare]

Run 'make distclean', and GNUmakefile is removed.
But, GNUmakefile is where we cd to build/.
Run 'make distclean' or 'make clean' again, and Makefile applies
the clean actions, such as this one, at the top level of the tree:

    find . \( -name '*.so' -o -name '*.dll' -o \
          -name '*.[oda]' -o -name '*.gcno' \) -type f \
        ! -path ./roms/edk2/ArmPkg/Library/GccLto/liblto-aarch64.a \
        ! -path ./roms/edk2/ArmPkg/Library/GccLto/liblto-arm.a \
        -exec rm {} +

For example, it removes the .d source files in 'meson/test cases/d/*/*.d'.
The damage could be worse in the future if more suffixes are cleaned.

I don't have a suggested fix.  Recursion and the GNUmakefile bootstrap
make it non-trivial.

- Steve