The signature of call16 is currently missing the "typ" parameter. This
CL fixes this. This wasn't caught by vet because call16 is defined by
macro expansion (see #17544), and we didn't notice the mismatch with
the other call* functions because call16 is defined only on 32-bit
architectures and lives alone in stubs32.go.

Unfortunately, this means its GC signature is also wrong: the "arg"
parameter is treated as a scalar rather than a pointer, so GC won't
trace it and stack copying won't adjust it. This turns out to matter
in exactly one case right now: on 32-bit architectures (which are the
only architectures where call16 is defined), a stack-allocated defer
of a function with a 16-byte or smaller argument frame including a
non-empty result area can corrupt memory if the deferred function
grows the stack and is invoked during a panic. Whew. All other current
uses of reflectcall pass a heap-allocated "arg" frame (which happens
to be reachable from other stack roots, so tracing isn't a problem).

Curiously, in 2016, the signatures of all call* functions were wrong
in exactly this way. CL 31654 fixed all of them in stubs.go, but
missed the one in stubs32.go.

Updates #41795.
Fixes #41796.

Change-Id: I31e3c0df201f79ee5707eeb8dc4ff0d13fc10ada
Reviewed-on: https://go-review.googlesource.com/c/go/+/259338
Trust: Austin Clements <austin@google.com>
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/259597