cmd/go, runtime: define GOBUILDMODE_shared rather than shared when dynamically linking

To avoid collisions with what existing code may already be doing.

Change-Id: Ice639440aafc0724714c25333d90a49954372230
Reviewed-on: https://go-review.googlesource.com/16503Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/build.go

@@ -389,7 +389,7 @@ func buildModeInit() {
 		} else {
 			switch platform {
 			case "linux/386", "linux/amd64", "linux/arm", "linux/arm64":
-				buildAsmflags = append(buildAsmflags, "-D=shared=1")
+				buildAsmflags = append(buildAsmflags, "-D=GOBUILDMODE_shared=1")
 			default:
 				fatalf("-buildmode=shared not supported on %s\n", platform)
 			}
@@ -408,7 +408,7 @@ func buildModeInit() {
 		} else {
 			switch platform {
 			case "linux/386", "linux/amd64", "linux/arm", "linux/arm64":
-				buildAsmflags = append(buildAsmflags, "-D=shared=1")
+				buildAsmflags = append(buildAsmflags, "-D=GOBUILDMODE_shared=1")
 			default:
 				fatalf("-buildmode=shared not supported on %s\n", platform)
 			}

src/runtime/sys_linux_386.s

@@ -10,7 +10,7 @@
 #include "go_tls.h"
 #include "textflag.h"
 
-#ifdef shared
+#ifdef GOBUILDMODE_shared
 #define INVOKE_SYSINFO CALL 0x10(GS)
 #else
 #define INVOKE_SYSINFO CALL *runtime·_vdso(SB)