Build failures with high parallel make(1) jobs with GCC

I have been pounding my head against the desk for longer than I care to
admit with this failure.

I see this with powerpc, powerpc64, and now ia64.  I initially thought
it was specific to powerpc{,64}, but now realize ia64 is also affected.

This build was running with -j48 on a 48-core machine, when the
following caused build failure:

=======================================================================

/usr/obj/ia64.ia64/usr/src/tmp/usr/bin/ld: cannot find -lm
--- libstdc++.so.6 ---
*** [libstdc++.so.6] Error code 1

make[4]: stopped in /usr/src/gnu/lib/libstdc++
A failure has been detected in another branch of the parallel make

=======================================================================

It is unclear to me when exactly this started happening, but it seems at
least two weeks is a reasonable estimate.

I realize this is not an entirely large chunk of useful information
regarding the build failure, but I have determined it is entirely
reproducible.  I have determined that the failure case seems to
disappear with make(1) jobs <= 10, at least for powerpc.

The last successful build for powerpc on head/ was April 8.  But I am
having trouble tracking down what commits may (or may not) have
contributed to recent high-parallel build failures.

Glen