Use read_volatile for stack_check_guard.

Its whole point is that it will be aliased if the stack overflows, so it
doesn't make sense to use a reference. In fact the compiler might
optimise away the reads. Reading a pointer is still a bit dodgy, but
better.

Test: m vmbase_example_bin
Change-Id: I23e83d2b83d945d0fe66bcbd909cd5e04a609e12
diff --git a/vmbase/example/src/main.rs b/vmbase/example/src/main.rs
index b3b5732..021daa4 100644
--- a/vmbase/example/src/main.rs
+++ b/vmbase/example/src/main.rs
@@ -25,7 +25,7 @@
 
 use crate::layout::{
     bionic_tls, boot_stack_range, dtb_range, print_addresses, rodata_range, scratch_range,
-    stack_chk_guard, text_range, DEVICE_REGION,
+    text_range, DEVICE_REGION,
 };
 use crate::pci::{check_pci, get_bar_region};
 use aarch64_paging::{idmap::IdMap, paging::Attributes};
@@ -33,7 +33,7 @@
 use fdtpci::PciInfo;
 use libfdt::Fdt;
 use log::{debug, error, info, trace, warn, LevelFilter};
-use vmbase::{configure_heap, cstr, logger, main, memory::SIZE_64KB};
+use vmbase::{configure_heap, cstr, layout::stack_chk_guard, logger, main, memory::SIZE_64KB};
 
 static INITIALISED_DATA: [u32; 4] = [1, 2, 3, 4];
 static mut ZEROED_DATA: [u32; 10] = [0; 10];