cmd/compile: split exported/non-exported methods for interface type
Currently, mhdr/methods is emitted with the same len/cap. There's no way
to distinguish between exported and non-exported methods statically.
This CL splits mhdr/methods into two parts, use "len" for number of
exported methods, and "cap" for all methods. This fixes the bug in
issue #22075, which intends to return the number of exported methods but
currently return all methods.
Note that with this encoding, we still can access either
all/exported-only/non-exported-only methods: