base: Add encapsulation to the loader::Symbol class

This commit converts `gem5::loader::Symbol` to a full class with
private members, enforcing encapsulation. Until now client code has
been able to (and does) access members directly.

This change will enable class invariants to be enforced via accessor
methods.

Change-Id: Ia0b5b080d4f656637a211808e13dce1ddca74541
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>

src/sim/kernel_workload.cc

@@ -66,8 +66,8 @@ KernelWorkload::KernelWorkload(const Params &p) : Workload(p),
         kernelSymtab = kernelObj->symtab();
         auto initKernelSymtab = kernelSymtab.mask(_loadAddrMask)
             ->offset(_loadAddrOffset)
-            ->rename([](std::string &name) {
-                name = "kernel_init." + name;
+            ->rename([](const std::string &name) {
+                return "kernel_init." + name;
             });
 
         loader::debugSymbolTable.insert(*initKernelSymtab);