cmd/internal/ld: put read-only relocated data into .data.rel.ro when making a shared object

Currently Go produces shared libraries that cannot be shared between processes
because they have relocations against the text segment (not text section). This
fixes this by moving some data to sections with magic names recognized by the
static linker.

The change in genasmsym to add STYPELINK to the switch should fix things on
darwin/arm64.

Fixes #10914
Updates #9210

Change-Id: Iab4a6678dd04cec6114e683caac5cf31b1063309
Reviewed-on: https://go-review.googlesource.com/14306
Run-TryBot: Michael Hudson-Doyle <michael.hudson@canonical.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/internal/obj/link.go

@@ -334,6 +334,7 @@ const (
 	Sxxx = iota
 	STEXT
 	SELFRXSECT
+
 	STYPE
 	SSTRING
 	SGOSTRING
@@ -341,6 +342,25 @@ const (
 	SGCBITS
 	SRODATA
 	SFUNCTAB
+
+	// Types STYPE-SFUNCTAB above are written to the .rodata section by default.
+	// When linking a shared object, some conceptually "read only" types need to
+	// be written to by relocations and putting them in a section called
+	// ".rodata" interacts poorly with the system linkers. The GNU linkers
+	// support this situation by arranging for sections of the name
+	// ".data.rel.ro.XXX" to be mprotected read only by the dynamic linker after
+	// relocations have applied, so when the Go linker is creating a shared
+	// object it checks all objects of the above types and bumps any object that
+	// has a relocation to it to the corresponding type below, which are then
+	// written to sections with appropriate magic names.
+	STYPERELRO
+	SSTRINGRELRO
+	SGOSTRINGRELRO
+	SGOFUNCRELRO
+	SGCBITSRELRO
+	SRODATARELRO
+	SFUNCTABRELRO
+
 	STYPELINK
 	SSYMTAB
 	SPCLNTAB