x86/Intel: restrict use of LONG_DOUBLE_MNEM_SUFFIX

As a comment near the top of match_template() already says: We really
only need this pseudo-suffix for far branch handling. Stop "deriving" it
for floating point insns. (Don't bother renaming the now properly
misnamed LONG_DOUBLE_MNEM_SUFFIX, to e.g. FAR_BRANCH_SUFFIX - it's going
to disappear anyway.)