Merge new LLVM patches

[brson]

We're starting to get a backlog

• MIPS stacks improve MIPS backend and implement segmented stacks #6358 http://people.cs.nctu.edu.tw/%7Ejyyou/rust/mips-segstk.patch
• .ll parsing Rebases llvm to near-head, fixes .ll parse errors with fixedstacksegment attribute brson/llvm#8
• ARM stacks preliminary Linux ARM support #6249 http://people.cs.nctu.edu.tw/%7Ejyyou/rust/arm-segstk.patch

[crabtw]

Add a command line option for setting the size of fixed-size stack segments
http://people.cs.nctu.edu.tw/%7Ejyyou/rust/fixed-stkseg.patch

[brson]

Testing these on my wip branch now.

[alexcrichton]

Also, with threadsafe-llvm being enabled soon, this patch should probably be included in the bunch.

[alexcrichton]

These are all included in #7115

[alexcrichton]

Closed in #7115