Refactor bionic includes into a single place

Bionic includes are the same on all architectures, modulo
architecture-specific includes.  Use a single function to populate the
list, passing in bionic's and the kernel's names for the architecture.

Also get rid of the ${LibcRoot} variable, it is not providing any value
and makes grepping harder.

Change-Id: I39e7907d312f52dd1378a3937ab1bcba12c4e97f
diff --git a/cc/arm_device.go b/cc/arm_device.go
index e985a38..dc8ba81 100644
--- a/cc/arm_device.go
+++ b/cc/arm_device.go
@@ -168,14 +168,7 @@
 	pctx.StaticVariable("armCflags", strings.Join(armCflags, " "))
 	pctx.StaticVariable("armLdflags", strings.Join(armLdflags, " "))
 	pctx.StaticVariable("armCppflags", strings.Join(armCppflags, " "))
-	pctx.StaticVariable("armIncludeFlags", strings.Join([]string{
-		"-isystem ${LibcRoot}/arch-arm/include",
-		"-isystem ${LibcRoot}/include",
-		"-isystem ${LibcRoot}/kernel/uapi",
-		"-isystem ${LibcRoot}/kernel/android/uapi",
-		"-isystem ${LibcRoot}/kernel/common",
-		"-isystem ${LibcRoot}/kernel/uapi/asm-arm",
-	}, " "))
+	pctx.StaticVariable("armIncludeFlags", bionicHeaders("arm", "arm"))
 
 	// Extended cflags