put __stl_prime_list in comdat section

[Xinliang David Li]

Hi, the following patch is a follow up to the one posted here
http://gcc.gnu.org/ml/gcc-patches/2009-05/msg01293.html.

The new patch is a header only change and can greatly reduce rodata
section size for some programs.

Ok for trunk after testing?

thanks,

David

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stl_prime.p
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