Rename `col!` to `column!`.

This macro is very rarely used, so there is no need (and it is better)
for it to avoid the abbreviation.

Closes rust-lang/rfcs#467.

[breaking-change]
auto merge of #19071 : huonw/rust/col2column, r=nikomatsakis

This macro is very rarely used, so there is no need (and it is better)
for it to avoid the abbreviation.

Closes rust-lang/rfcs#467.
mk/stage0: complain instead of creating an empty file

If the expected rustc snapshot is not where we expect it to be,
complain and fail at that point rather than creating a empty rustc file
and continuing until we try to run it.
mk/target: fix typo so we depend on the correct directory

Without this, if we we're using a non-standard host libdir, the target
bindir would not exist (and rustc would fail to write to the
non-existent directory).
rustdoc: avoid supplying a bad default sysroot so the librustc code can calculate it properly
configure: silence warning about LOCAL_RUST_ROOT being set

We have a default value for this ('/usr/local'), so this warning is
printed ALL the time unless one does --enable-local-rust. As a result,
it doesn't really help at all.
mk/rt: use CFG_LLVM_TARGET instead of plain target when calling llc