Change the way libc++ is linked when use_custom_libcxx=1.

Change the way libc++ is linked when use_custom_libcxx=1.

Instead of passing libc++.so and libc++abi.so on the command line to the linker,
we want to be able to simply pass -stdlib=libc++ to the clang driver and let it
make the decision about linking the DSOs. In this way we can avoid linking
libc++ into C executables.

To this effect we introduce a proxy target which adds -stdlib=libc++ to direct
dependents' flags but does not link the actual DSO. We also link libc++abi
statically into libc++. The latter change is necessary because clang's
-stdlib=libc++ mode is set up to use libsupc++ as the ABI implementation, and
will not automatically pick up libc++abi.so.

BUG=318770
R=glider@chromium.org, thakis@chromium.org
NOTRY=true
TEST=msan

Committed: https://src.chromium.org/viewvc/chrome?view=rev&revision=268169

[earthdok, 2014-04-30 17:25:43]

ptal

Nico - /base/ + FYI on the security issue in libc++.gyp
Alex - everything else

[Alexander Potapenko, 2014-04-30 18:18:38]

Looks reasonable
lgtm

https://codereview.chromium.org/264613005/diff/20001/third_party/libc++/libc+...
File third_party/libc++/libc++.gyp (right):

https://codereview.chromium.org/264613005/diff/20001/third_party/libc++/libc+...
third_party/libc++/libc++.gyp:51: # Libc++abi is linked statically into
libc++.so. This allows us to get
s/L/l

[Nico, 2014-04-30 18:19:20]

lgtm

[earthdok, 2014-05-05 11:45:23]

The CQ bit was checked by earthdok@chromium.org

[commit-bot: I haz the power, 2014-05-05 11:45:33]

CQ is trying da patch. Follow status at
 https://chromium-status.appspot.com/cq/earthdok@chromium.org/264613005/30001

[commit-bot: I haz the power, 2014-05-05 13:30:49]

Change committed as 268169