[This CL is part of the removal of C code from package runtime.
See golang.org/s/dev.cc for an overview.]

export.c, lex.c:
Add -asmhdr flag to write assembly header file with struct
field offsets and const values. cmd/dist used to construct this
file by interpreting output from the C compiler.
Generate it from the Go definitions instead.
Also, generate the form we need directly, instead of relying
on cmd/dist for reprocessing.

lex.c, obj.c:
If the C compiler accepted #pragma cgo_xxx, recognize
a directive //go:cgo_xxx instead. The effect is the same as
in the C compiler: accumulate text into a buffer and emit in the
output file, where the linker will find and use it.

lex.c, obj.c:
Accept //go:linkname to control the external symbol name
used for a particular top-level Go variable. This makes it
possible to refer to C symbol names but also symbols from
other packages. It has always been possible to do this from
C and assembly. To drive home the point that this should not
be done lightly, require import "unsafe" in any file containing
//go:linkname.

plive.c, reflect.c, subr.c:
Hard-code that interfaces contain only pointers.
This means code handling multiword values in the garbage
collector and the stack copier can be deleted instead of being
converted. This change is already present in the dev.garbage
branch.

LGTM=r
R=r
CC=austin, golang-codereviews, iant, khr
https://golang.org/cl/169360043