ctags: add macros
librustc: don't skip ahead when computing register types of SSEFloatVectors

Also fixes an unrelated typo (found by comparing to the original code
of the clay project) and some cleanup.

Fixes #7415.
auto merge of #7457 : Blei/rust/fix-ffi-floats, r=cmr

Also contains a fix to help ctag pick up macro definitions.