runtime: add -buildmode=c-archive/c-shared support for linux/386

Change-Id: I87147ca6bb53e3121cc4245449c519509f107638
Reviewed-on: https://go-review.googlesource.com/9009
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/dist/test.go

@@ -378,7 +378,7 @@ func (t *tester) buildmode(mode string) bool {
 		case t.goos == "darwin" && t.goarch == "amd64":
 			// TODO(crawshaw): add darwin/arm{,64}
 			return true
-		case t.goos == "linux" && t.goarch == "amd64":
+		case t.goos == "linux" && (t.goarch == "amd64" || t.goarch == "386"):
 			return true
 		default:
 			return false

src/runtime/rt0_linux_386.s

@@ -12,6 +12,47 @@ TEXT _rt0_386_linux(SB),NOSPLIT,$8
 	CALL	main(SB)
 	INT	$3
 
+// When building with -buildmode=c-shared, this symbol is called when the shared
+// library is loaded.
+TEXT _rt0_386_linux_lib(SB),NOSPLIT,$12
+	MOVL	16(SP), AX
+	MOVL	AX, _rt0_386_linux_lib_argc<>(SB)
+	MOVL	20(SP), AX
+	MOVL	AX, _rt0_386_linux_lib_argv<>(SB)
+
+	// Create a new thread to do the runtime initialization.
+	MOVL	_cgo_sys_thread_create(SB), AX
+	TESTL	AX, AX
+	JZ	nocgo
+	MOVL	$_rt0_386_linux_lib_go(SB), BX
+	MOVL	BX, 0(SP)
+	MOVL	$0, 4(SP)
+	CALL	AX
+	RET
+
+nocgo:
+	MOVL	$0x800000, 0(SP)                    // stacksize = 8192KB
+	MOVL	$_rt0_386_linux_lib_go(SB), AX
+	MOVL	AX, 4(SP)                           // fn
+	MOVL	$0, 8(SP)                           // fnarg
+	MOVL	$runtime·newosproc0(SB), AX
+	CALL	AX
+	RET
+
+TEXT _rt0_386_linux_lib_go(SB),NOSPLIT,$12
+	MOVL	_rt0_386_linux_lib_argc<>(SB), AX
+	MOVL	AX, 0(SP)
+	MOVL	_rt0_386_linux_lib_argv<>(SB), AX
+	MOVL	AX, 4(SP)
+	MOVL	$runtime·rt0_go(SB), AX
+	CALL	AX
+	RET
+
+DATA _rt0_386_linux_lib_argc<>(SB)/4, $0
+GLOBL _rt0_386_linux_lib_argc<>(SB),NOPTR, $4
+DATA _rt0_386_linux_lib_argv<>(SB)/4, $0
+GLOBL _rt0_386_linux_lib_argv<>(SB),NOPTR, $4
+
 TEXT main(SB),NOSPLIT,$0
 	JMP	runtime·rt0_go(SB)