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/uefi/bootstrap_uefi.c

@@ -35,7 +35,12 @@
 
 void start(void);
 
-EFI_STATUS EFIAPI
+/* The section attribute below is copied from ATTR_BOOT_CODE in prot-domains.h.
+ * prot-domains.h includes stdlib.h which defines NULL.  The UEFI headers also
+ * define NULL, which induces a warning when the compiler detects the conflict.
+ * To avoid that, we avoid including prot-domains.h from this file.
+ */
+EFI_STATUS EFIAPI __attribute__((section(".boot_text")))
 uefi_start(IN EFI_HANDLE ImageHandle, IN EFI_SYSTEM_TABLE *SystemTable)
 {
   EFI_MEMORY_DESCRIPTOR mem_map[MAX_MEM_DESC];