runtime: mark all Go symbols called from assembly in other packages

This marks all Go symbols called from assembly in other packages with
"go:linkname" directives to ensure they get ABI wrappers.

Now that we have this go:linkname convention, this also removes the
abi0Syms definition in the runtime, which was used to give morestackc
an ABI0 wrapper. Instead, we now just mark morestackc with a
go:linkname directive.

This was tested with buildall.bash in the default configuration, with
-race, and with -gcflags=all=-d=ssa/intrinsics/off. Since I couldn't
test cgo on non-Linux configurations, I manually grepped for runtime
symbols in runtime/cgo.

Updates #31230.

Change-Id: I6c8aa56be2ca6802dfa2bf159e49c411b9071bf1
Reviewed-on: https://go-review.googlesource.com/c/go/+/179862
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/src/runtime/string.go b/src/runtime/string.go
index 839e882cdc..d198f73756 100644
--- a/src/runtime/string.go
+++ b/src/runtime/string.go
@@ -310,6 +310,8 @@ func gobytes(p *byte, n int) (b []byte) {
 	return
 }
 
+// This is exported via linkname to assembly in syscall (for Plan9).
+//go:linkname gostring
 func gostring(p *byte) string {
 	l := findnull(p)
 	if l == 0 {