cmd/go,cmd/link: prefer external linking when strange cgo flags seen

This patch changes the Go command to examine the set of compiler
flags feeding into the C compiler when packages that use cgo are built.
If any of a specific set of strange/dangerous flags are in use,
then the Go command generates a token file ("preferlinkext") and
embeds it into the compiled package's archive.

When the Go linker reads the archives of the packages feeding into the
link and detects a "preferlinkext" token, it will then use external
linking for the program by default (although this default can be
overridden with an explicit "-linkmode" flag).

The intent here is to avoid having to teach the Go linker's host object
reader to grok/understand the various odd symbols/sections/types that
can result from boutique flag use, but rather to just boot the objects
in question over to the C linker instead.

Updates #58619.
Updates #58620.
Updates #58848.

Change-Id: I56382dd305de8dac3841a7a7e664277826061eaa
Reviewed-on: https://go-review.googlesource.com/c/go/+/475375
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

1 files changed, 2 insertions, 0 deletions

diff --git a/src/cmd/go/testdata/script/README b/src/cmd/go/testdata/script/README
index 7b747994c6..349ba972fb 100644
--- a/src/cmd/go/testdata/script/README
+++ b/src/cmd/go/testdata/script/README
@@ -382,6 +382,8 @@ The available conditions are:
 	$WORK filesystem is case-sensitive
 [cgo]
 	host CGO_ENABLED
+[cgolinkext]
+	platform requires external linking for cgo
 [compiler:*]
 	runtime.Compiler == <suffix>
 [cross]