MCAsmParser: add support for .err directive

The .err directive produces an error whenever it is assembled.  This can be
useful for preventing assembly when an unexpected condition occurs.
MCAsmParser: handle space properly for .ifc/.ifnc

If the strings are not quoted, the first string stops at the first comma, and
the second string stops at the end of the line.  Strings which contain
whitespace should be quoted.  Unquoted space is to be discarded.
MCAsmParser: support .ifne

The .ifne directive assembles the following section of code if the argument
expression is non-zero.  Effectively, it is equivalent to if.