rustc: Don't always build a constant multiply instruction when translating literal vectors; the size of the referent isn't always constant

Make some standard library pieces no longer dependent on mutable parameters, which rustc doesn't support

rustc: Add str_from_cstr() and str_from_buf() functions to the standard library, as well as a test case

Add a "rustllvm.def" file for Windows' linker to use. This allows us to create our own LLVM shared library on that platform.

Fixed version of my previous patch to rename the upcall functions. This time also update the inline assembly.

Switch all vases of vec += elt to vec += vec. Prohibit former in rustboot. Tweak std lib vec fns in process.