x86: Add support for SW-switched segment-based protection domains

This patch extends the protection domain framework with a third plugin
that is a hybrid of the previous two.  The hardware task switching
mechanism has a strictly-defined format for TSS data structures that
causes more space to be consumed than would otherwise be required.
This patch defines a smaller data structure that is allocated for each
protection domain, only requiring 32 bytes instead of 128 bytes.  It
uses the same multi-segment memory layout as the TSS-based plugin and
leaves paging disabled.  However, it uses a similar mechanism as the
paging plugin to perform system call dispatches and returns.

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

cpu/x86/mm/syscalls-int.h

@@ -74,7 +74,7 @@ extern dom_id_t cur_dom;
     #nm ":\n\t"                                                               \
     /* First, load server protection domain ID into EDX, as required by */    \
     /* prot_domains_syscall_dispatcher: */                                    \
-    "  mov " #dcd ", %edx\n\t"                                                \
+    "  mov %" SEG_KERN "s:" #dcd ", %edx\n\t"                                 \
     SYSCALLS_STUB_EPILOGUE(nm))
 
 void syscalls_int_init(void);