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Thanks for the details. So the relevant code (see the float_hex function in Objects/floatobject.c) looks like this:
 if (x == 0.0) {
 if(copysign(1.0, x) == -1.0)
 return PyString_FromString("-0x0.0p+0");
 else
 return PyString_FromString("0x0.0p+0");
 }
This *should* produce the correct string for -0.0 (because -0.0 compares equal to 0.0, and then copysign(1.0, x) should be -1.0); I'm reasonably confident that the C code is correct, since the tests pass on all the other platforms that get tested regularly.
So a buggy system copysign function looks like a possibility. Another more likely possibility occurs to me, though: and that's that there's a buggy compiler optimization going on: the compiler sees that we're in an 'x == 0.0' branch, and decides that it can substitute '0.0' for 'x' everywhere in the 'if' block. But this is just guessing.
Do you still get these failures in a debug build of Python (i.e., by passing --with-pydebug to the configure script)?