arch/x86: add support for cache-as-ram paging

Processors, such as glk, need to have paging enabled while
in cache-as-ram mode because the front end is agressive about
fetching lines into the L1I cache. If the line is dirty and in
the L1D then it writes it back to "memory". However, in this case
there is no backing store so the cache-as-ram data that was written
back transforms to all 0xff's when read back in causing corruption.

In order to mitigate the failure add x86 architecture support for
enabling paging while in cache-as-ram mode. A Kconfig variable,
NUM_CAR_PAGE_TABLE_PAGES, determines the number of pages to carve
out for page tables within the cache-as-ram region. Additionally,
the page directory pointer table is also carved out of cache-as-ram.
Both areas are allocated from the persist-across-stages region
of cache-as-ram so all stages utilizing cache-as-ram don't corrupt
the page tables.

The two paging-related areas are loaded by calling
paging_enable_for_car() with the names of cbfs files to load the
initial paging structures from.

BUG=b:72728953

Change-Id: I7ea6e3e7be94a0ef9fd3205ce848e539bfbdcb6e
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/25717
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Justin TerAvest <teravest@chromium.org>

src/include/symbols.h

@@ -106,6 +106,10 @@ extern u8 _framebuffer[];
 extern u8 _eframebuffer[];
 #define _framebuffer_size (_eframebuffer - _framebuffer)
 
+extern u8 _pdpt[];
+extern u8 _epdpt[];
+#define _pdpt_size (_epdpt - _pdpt)
+
 /* Put this into a .c file accessing a linker script region to mark that region
  * as "optional". If it is defined in memlayout.ld (or anywhere else), the
  * values from that definition will be used. If not, start, end and size will