x86: honor quoted figure braces in i386_att_operand()

x86: honor quoted figure braces in i386_att_operand()

When AVX512 support was added, symbol quotation was not paid attention
to. Just like the (base,index,scale) specifier gets parsed from the end
of the expression, the {...} also wants parsing from the end; in no case
is the first { found a guarantee of a masking or broadcasting specifier.