x86: avoid strcmp() in a few places

x86: avoid strcmp() in a few places

Now that we have identifiers for the mnemonic strings we can avoid
strcmp() in a number of places, comparing the offsets into the mnemonic
string table instead. While doing this also
- convert a leftover strncmp() to startswith() (apparently an oversight
  when rebasing the original patch introducing the startswith() uses),
- use the new shorthand for current_templates->start also elsewhere in
  md_assemble() (valid up to the point where match_template() is
  called).