[driver] Add configure-time specs

[Nathan Sidwell]

Nathan Sidwell wrote:
> This patch of Mark's adds a --with-specs=... configure option to gcc.  
> This allows you to tune a toolchain build without needing to add or 
> adjust various t-  fragments or header files.  We use it to give fine 
> grained control over optimization levels, amongst other things.
> 
> built and tested (with and without the config option) on 
> i686-pc-linux-gnu. ok?

ping?

nathan

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