[Patch] GettingStarted.rst update

[Erik de Castro Lopo]

Hi,

Two GettingStarted.rst patches attached.

* Explain what compiler-rt does and why it may be needed.
* Add libcxx and libcxxabi to the build.

Patches were validated using `make -f Makefile.sphinx` as suggested
by @bogner on IRC.

Cheers,
Erik
-- 
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Erik de Castro Lopo
http://www.mega-nerd.com/
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