gnu: gcc: Fix cross-compilation.

gnu: gcc: Fix cross-compilation.

Previously "guix build gcc@5 --target=TRIPLET" would fail like this:

   checking for exported symbols... /tmp/guix-build-gcc-5.5.0.drv-0/gcc-5.5.0/libcc1/configure: line 14531: -T: command not found
   yes
   checking for -rdynamic... /tmp/guix-build-gcc-5.5.0.drv-0/gcc-5.5.0/libcc1/configure: line 14541: -T: command not found
   no
   checking for library containing dlopen... -ldl
   checking for -fPIC -shared... yes
   configure: error:
      Building GCC with plugin support requires a host that supports
      -fPIC, -shared, -ldl and -rdynamic.

* gnu/packages/gcc.scm (gcc-4.7)[maybe-target-tools]: Add "OBJDUMP".
(gcc-5)[source](snippet, modules): New fields.