[libc++][patch] Sort out common Linux linking issues.

[Sebastian Redl]

Following the recent inquiry and David Chisnall's advice, I have created
a patch that makes libc++ work a lot smoother on Linux. It allows a
plain "clang++ -stdlib=libc++" command line to work no matter what ABI
library is used, and fixes the linking errors I encountered.

However, I have absolutely no idea what it does on other platforms.

Log Message:

    Sort out common Linux linking issues.
   
    Using libc++ on Linux was problematic because using libsupc++ didn't
pull in
    all necessary symbols, whereas linking against a dynamic ABI library
required
    the user to explicitly link against the ABI library.
   
    This patch adds a new property, LIBCXX_CXX_ABI_LINK, that can be set
to dynamic
    (the default) or static. In dynamic mode, it links against the
dynamic library,
    and substitutes its unversioned .so file with a linker script that links
    against both libc++ and the ABI library. In static mode, it uses
--whole-archive
    to force inclusion of the entire ABI library into the libc++.so.
   
    It also automatically uses libstdc++ for dynamic mode and libsupc++
for static
    mode if either was selected; the two ABI names are now synonyms.
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