x86: Add support for (paging-based) protection domains

This patch implements a simple, lightweight form of protection domains
using a pluggable framework.  Currently, the following plugin is
available:

 - Flat memory model with paging.

The overall goal of a protection domain implementation within this
framework is to define a set of resources that should be accessible to
each protection domain and to prevent that protection domain from
accessing other resources.  The details of each implementation of
protection domains may differ substantially, but they should all be
guided by the principle of least privilege.  However, that idealized
principle is balanced against the practical objectives of limiting the
number of relatively time-consuming context switches and minimizing
changes to existing code.

For additional information, please refer to cpu/x86/mm/README.md.

This patch also causes the C compiler to be used as the default linker
and assembler.

cpu/x86/helpers.h

@@ -37,6 +37,17 @@
 
 void halt(void) __attribute__((__noreturn__));
 
+#define STRINGIFY(x) #x
+/* The C preprocessor will not expand macro arguments that are converted to
+ * strings in the macro body using the '#' operator.  The EXP_STRINGIFY macro
+ * introduces an additional level of argument expansion for instances where
+ * the developer wishes to convert the expanded argument to a string.
+ */
+#define EXP_STRINGIFY(x) STRINGIFY(x)
+
+#define ALIGN(x, amt) \
+  (((x) & ~((amt) - 1)) + ((((x) & ((amt) - 1)) == 0) ? 0 : (amt)))
+
 /** Wrappers for the assembly 'out' instruction. */
 void outb(uint16_t port, uint8_t val);
 void outl(uint16_t port, uint32_t val);