remove unused complicated arm variant enums, add a new simple one
arm: implement mla/mls pattern matching
remove arm_Mla_t node

You can simply build a Add(Mul,...) pattern now as codeselection will match
this to an Mla now.
arm: cleanup+reorg architecture variant options and init
arm: add pkhtb/pkhbt matcher
arm: skip sign convs
doc: Correct wrong s/error.h/panic.h/ in comments.

People, check your diffs and escape your dots!
be: Give the proj enums their names back.
be: Give the backend node operand enums a name.
amd64: Added generation of float constants