call dsymutil on darwin when compiling and linking in single step

[Jack Howarth]

Geoff,
   Does the patch you propose in...

http://gcc.gnu.org/ml/gcc-patches/2006-09/msg00057.html

eliminate the "can't find atom for N_GSYM stabs" warnings
in ld64? I ask because you that...

This is necessary since otherwise the debugger expects to find the
DWARF information in the .o file, but the compiler will have deleted
it.

...which makes me wonder if this could be the case.
                Jack