gcc: for cross compilers, don't build libgcc twice

Cross-compiled binaries currently end up with two different libgcc
outpaths in their closure.  This is harmless, but confusing.

The two libgccs are:

- One of them is the "first" targetPlatform libgcc, which is built
  by the "first" cross-compiler.  This "first libgcc" and "first
  compiler" are used to build the targetPlatform glibc.

- Once glibc is built, we *rebuild* the cross-compiler, since gcc
  can't enable most of its features unless you give it an
  already-compiled targetPlatform glibc.  When this "second"
  compiler is built, it also builds an extra copy of libgcc.

This commit discards the second, extra libgcc, and instead puts a
reference to the first (correct) libgcc into the "second compiler"
`.passthru.libgcc`, so that anybody expecting `stdenv.cc.cc.libgcc`
to exist will still find it there.

Closes #249680

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