Match Rust gcda output to cc via -Z profile-emit.

Use the -Z profile-emit flag to ensure that rust gcda files have
/proc/self/cwd/ appended to them similar to cc modules. This makes sure
our gcda output is consistent no matter what language the binary is
written in.

Bug: 156482307
Test: gcda outputs to the new path.

Change-Id: Ife4d55f885c7e33dffa66f7436bc2bf8b5916586
diff --git a/cc/builder.go b/cc/builder.go
index e571e5a..b4f9947 100644
--- a/cc/builder.go
+++ b/cc/builder.go
@@ -283,16 +283,19 @@
 		"cFlags")
 )
 
+func PwdPrefix() string {
+	// Darwin doesn't have /proc
+	if runtime.GOOS != "darwin" {
+		return "PWD=/proc/self/cwd"
+	}
+	return ""
+}
+
 func init() {
 	// We run gcc/clang with PWD=/proc/self/cwd to remove $TOP from the
 	// debug output. That way two builds in two different directories will
 	// create the same output.
-	if runtime.GOOS != "darwin" {
-		pctx.StaticVariable("relPwd", "PWD=/proc/self/cwd")
-	} else {
-		// Darwin doesn't have /proc
-		pctx.StaticVariable("relPwd", "")
-	}
+	pctx.StaticVariable("relPwd", PwdPrefix())
 
 	pctx.HostBinToolVariable("SoongZipCmd", "soong_zip")
 	pctx.Import("android/soong/remoteexec")