rustc: Pass -dead_strip on OSX

This flag to the linker asks it to strip away all dead code during linking, as
well as dead data. This reduces the size of hello world from 1.7MB to 458K on my
system (70% reduction).

I have not seen this impact link times negatively, and I have seen this pass
'make check' successfully. I am slightly wary of adding this option, but the
benefits are so huge tha I think we should work hard to work around any issues
rather than avoid using the flag entirely.
rustdoc: #13771: Make html links inside paragraphs more readable

All links inside docblocks will have their color set to `#4e8bca` (a
light blue color to contrast against the black text). This color also
offers a visible contrast from the surrounding text if viewed as
grayscale, making it suitable for accessability.

Docblock links will also be underlined when hovered over.
auto merge of #13829 : alexcrichton/rust/dead-strip, r=thestinger

This flag to the linker asks it to strip away all dead code during linking, as
well as dead data. This reduces the size of hello world from 1.7MB to 458K on my
system (70% reduction).

I have not seen this impact link times negatively, and I have seen this pass
'make check' successfully. I am slightly wary of adding this option, but the
benefits are so huge tha I think we should work hard to work around any issues
rather than avoid using the flag entirely.
auto merge of #13807 : ipetkov/rust/issue-13771, r=alexcrichton

All links inside docblocks will have their color set to `#4e8bca` (a light blue color to contrast against the black text). This color also offers a visible contrast from the surrounding text if viewed as grayscale, making it suitable for accessability.
    
Docblock links will also be underlined when hovered over.

Before:
![screen shot 2014-04-27 at 12 47 06 pm](https://cloud.githubusercontent.com/assets/1638690/2812157/00e53a32-ce45-11e3-81e8-7b1dc692f6b7.png)



After (links underlined only on hover, cursor not shown in image):
![screen shot 2014-04-27 at 12 47 48 pm](https://cloud.githubusercontent.com/assets/1638690/2812158/04fa94b4-ce45-11e3-9ead-2344753c251d.png)

Fix #13771