MFC r257658 (modified to apply to stable/7):

For C++ programs, don't emit any explicit dependency to libstdc++.a or
libc++.a during the early build stages (bootstrap-tools, build-tools,
cross-tools), since it is not possible to know in advance which C++
library is available on the host system.

Instead, just use the bootstrap compiler's built-in default.  This
should eventually make it possible to build stable/9 on head, or on
stable/10, which ship without libstdc++ by default.

[Note: this should get the stable/7 tinderbox at least a bit further in
buildworld.]