Use SRCDIR as a working directory

The existing behavior of using the build directory as the working
directory is useful if you want to move/copy the output directory around
and SRCDIR still refers the the source. But, it's more useful to have
the source directory be the working directory. Tools like cpp(__FILE__)
and other debug prints embed relative paths from the working directory.
We also have tools that expect the working directory to be $TOP.

Change-Id: Ia0f1d3c6b7df72d61cf5628efa2baa98bd19775b
diff --git a/cc/cc.go b/cc/cc.go
index 398a065..34a1a56 100644
--- a/cc/cc.go
+++ b/cc/cc.go
@@ -64,8 +64,8 @@
 	HostPrebuiltTag = pctx.VariableConfigMethod("HostPrebuiltTag", common.Config.PrebuiltOS)
 	SrcDir          = pctx.VariableConfigMethod("SrcDir", common.Config.SrcDir)
 
-	LibcRoot = pctx.StaticVariable("LibcRoot", "${SrcDir}/bionic/libc")
-	LibmRoot = pctx.StaticVariable("LibmRoot", "${SrcDir}/bionic/libm")
+	LibcRoot = pctx.StaticVariable("LibcRoot", "bionic/libc")
+	LibmRoot = pctx.StaticVariable("LibmRoot", "bionic/libm")
 )
 
 // Flags used by lots of devices.  Putting them in package static variables will save bytes in