x86: accept whitespace inside curly braces

Other than documented /**/ comments currently aren't really converted to
a single space, at least not for x86 in its most common configurations.
That'll be fixed subsequently, at which point blanks may appear where so
far none were expected. Furthermore not permitting blanks immediately
inside curly braces wasn't quite logical anyway - such constructs are
composite ones, and hence components ought to have been permitted to be
separated by whitespace from the very beginning.

With this we also don't care anymore whether the scrubber would remove
whitespace around curly braces, so move them from extra_symbol_chars[]
to operand_special_chars[].

Note: The new testcase doesn't actually exercise much (if any) of the
added code. It is being put in place to ensure that subsequently, when
that code actually comes into play, behavior remains the same.