be: Add detailed panic messages about missing transformers to node and Proj transform helpers.

This is useful for backend development.
ir: Let get_irg_anchor() just return the anchor instead of its operands.

This also renders get_irg_n_anchors() obsolete, so remove it.
ir: Specify the operands of Anchor.
be: Simplify initialisation and evaluation of asm constraints.
be: Correct handling of (and ignore) the '*' asm constraint modifier.
be: Only accept the '=' and '+' asm constraint modifiers at the beginning and only one of them.

* GCC does this, too.
* This is sensible, because they apply to all alternatives of a multi-alternative constraint.
* This simplifies the implementation and makes some flags obsolete.
be: Slighty improve handling of multi-alternative constraints.

* Classify ',' as "no support" instead of "invalid".
* Ignore '!' and '?'.
ia32: Use an enum bitfield for the attribute tp of ia32_attr_t.

* Also only use the necessary number of bits.
* Improve its comment.
ia32: Factorise emission of immediates in binary emitter.
ia32: Cleanup in the binary emitter.
Sort git attribute list.
Ignore some files when using git archive.