A machine description has two parts: a file of instruction patterns
.md file) and a C header file of macro definitions.
.md file for a target machine contains a pattern for each
instruction that the target machine supports (or at least each instruction
that is worth telling the compiler about). It may also contain comments.
A semicolon causes the rest of the line to be a comment, unless the semicolon
is inside a quoted string.
See the next chapter for information on the C header file.