gccCrossStageStatic: enable dynamic libraries, rename to gccWithoutTargetLibc

This commit allows `gccCrossStageStatic` to build dynamically-linked
libraries.  Since is no longer restricted to building static
libraries its name is no longer appropriate, and this commit also
renames it to the more-accurate `gccWithoutTargetLibc`.

By default, you can't build a gcc that knows how to create dynamic
libraries unless you have already built the targetPlatform libc.

Because of this, our gcc cross-compiler is built in two stages:

  1. Build a cross-compiler (gccCrossStageStatic) that can build
     only static libraries.

  2. Use gccCrossStageStatic to compile the targetPlatform libc.

  3. Use the targetPlatform libc to build a fully-capable cross
     compiler.

You might notice that this pattern looks very similar to what we do
with `xgcc` in the stdenv bootstrap.  Indeed it is!  I would like to
work towards getting the existing stdenv bootstrap to handle cross
compilers as well.  However we don't want to cripple `stdenv.xgcc`
by taking away its ability to build dynamic libraries.

It turns out that the only thing gcc needs the targetPlatform libc
for is to emit a DT_NEEDED for `-lc` into `libgcc.so`.  That's it!
And since we don't use `gccCrossStageStatic` to build anything other
than libc, it's safe to omit the `DT_NEEDED` because that `libgcc`
will never be loaded by anything other than `libc`.  So `libc` will
already be in the process's address space.

Other people have noticed this; crosstool-ng has been using this
approach for a very long time:

  https://github.com/crosstool-ng/crosstool-ng/blob/36ad0b17a732aaffe4701d5d8d410d6e3e3abba9/scripts/build/cc/gcc.sh#L638-L640

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