[MC] Add parseEOL() overload and migrate some parseToken(AsmToken::EndOfStatement) to parseEOL()

For many directives, the following diagnostics

* `error: unexpected token`
* `error: unexpected token in '.abort' directive"`

are replaced with `error: expected newline`.

`unexpected token` may make the user think a different token is needed.
`expected newline` is clearer about the expected token.

For `in '...' directive`, the directive name is not useful because the next line
replicates the error line which includes the directive.