crucible-llvm-syntax: Concrete syntax for `Alignment`

[langston-barrett]

#1113 introduced support for parsing a single kind of Alignment value, namely none:

crucible/crucible-llvm-syntax/src/Lang/Crucible/LLVM/Syntax.hs

Line 174 in e9b1a01

unless (s == Atom.AtomName "none") Parse.cut

We should extend this to support all valid alignments.