ld: bump pe linker version to 3.0 to allow code signing

The Windows signtool.exe thinks our binaries are 'invalid
Win32 programs' unless the PE linker version field is 3.0
or greater.

This minor change makes it possible to successfully sign
gc-built binaries on Windows.

R=golang-dev, alex.brainman, rsc
CC=golang-dev
https://golang.org/cl/5268045

src/cmd/ld/pe.c

@@ -620,7 +620,7 @@ asmbpe(void)
 		set(Magic, 0x10b);	// PE32
 		oh.BaseOfData = d->VirtualAddress;
 	}
-	set(MajorLinkerVersion, 1);
+	set(MajorLinkerVersion, 3);
 	set(MinorLinkerVersion, 0);
 	set(SizeOfCode, t->SizeOfRawData);
 	set(SizeOfInitializedData, d->SizeOfRawData);