builder,loader: fix -ldflags -X not overriding variables with default values

When a string variable already had a source-level default value, the -ldflags
"-X" flag was silently ignored and the variable remained the default value
at runtime.

Fix by stripping the InitOrder entry for each -X variable before
LoadSSA() is called. With no entry in InitOrder, go/ssa emits no init
store, so the global stays zero-valued in the IR. makeGlobalsModule
still injects the actual -X values at final link time.

The -X values remain out of the per-package build cache with only the
variable names appearing in the cache key.

Signed-off-by: deadprogram <ron@hybridgroup.com>

testdata/ldflags-initialized.go

@@ -0,0 +1,9 @@
+package main
+
+// This global has a default value. It should be overridable via
+// -ldflags="-X main.someGlobal=value" just like an uninitialized global.
+var someGlobal = "default"
+
+func main() {
+	println("someGlobal:", someGlobal)
+}

testdata/ldflags-initialized.txt

@@ -0,0 +1 @@
+someGlobal: foobar

testdata/ldflags.go

@@ -1,7 +1,6 @@
 package main
 
-// These globals can be changed using -ldflags="-X main.someGlobal=value".
-// At the moment, only globals without an initializer can be replaced this way.
+// This global can be changed using -ldflags="-X main.someGlobal=value".
 var someGlobal string
 
 func main() {