Currently, tail calls on x86 don't adjust the SP on return, so it's
important that the compiler produce a zero-sized frame and disable the
frame pointer. However, these constraints aren't necessary. For
example, on other architectures it's generally necessary to restore
the saved LR before a tail call, so obj simply makes this work.
Likewise, on x86, there's no reason we can't simply make this work.

Hence, this CL adjusts the compiler to use the same tail call
convention for x86 that we use on LR machines by producing a RET with
a target, rather than a JMP with a target. In fact, obj already
understands this convention for x86 except that it's buggy with
non-zero frame sizes. So we also fix this bug obj. As a result of
these fixes, the compiler no longer needs to mark wrappers as
NoFramePointer since it's now perfectly fine to save the frame
pointer.

In fact, this eliminates the only use of NoFramePointer in the
compiler, which will enable further cleanups.

This also fixes what is very nearly, but not quite, a code generation
bug. NoFramePointer becomes obj.NOFRAME in the object file, which on
ppc64 and s390x means to omit the saved LR. Hence, on these
architectures, NoFramePointer (and NOFRAME) is only safe to set on
leaf functions. However, on *most* architectures, wrappers aren't
necessarily leaf functions because they may call DUFFZERO. We're saved
on ppc64 and s390x only because the compiler doesn't have the rules to
produce DUFFZERO calls on these architectures. Hence, this only works
because the set of LR architectures that implement NOFRAME is disjoint
from the set where the compiler produces DUFFZERO operations. (I
discovered this whole mess when I attempted to add NOFRAME support to
arm.)

Change-Id: Icc589aeb86beacb850d0a6a80bd3024974a33947
Reviewed-on: https://go-review.googlesource.com/92035
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>