compiler: make value method of direct interface type takes pointer

Currently, a value method of a direct interface type takes the
value of the receiver, which is pointer shaped, as the first
parameter. When this method is called through interface, we
actually pass the interface data as a pointer. On most platforms
this is ok, as the underlying calling convention is the same,
except that on SPARC32, the calling convention is actually
different.

This CL changes the method function actually takes a pointer.
The function will convert the pointer to the pointer-shaped
receiver type (a no-op conversion from machine code's aspect).
For a direct call, in the caller we convert the receiver to a
pointer (also no-op conversion) before invoking the method. For
an interface call, we pass the pointer as before. This way, it is
consistent that we always pass a pointer.

Hopefully this fixes SPARC32 build and https://gcc.gnu.org/PR90482.

Change-Id: I67808afcf847fb2ebe2bbcf74df158e7887f81cd
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/177758
Reviewed-by: Ian Lance Taylor <iant@golang.org>
4 files changed