arch/x86: allow idt to be available to link in all stages

Add Kconfig IDT_IN_EVERY_STAGE to optionally specify having
the interrupt handling code available to all stages. In order
to do this the idt setup is moved to a C module. The vecX
entries are made global so that a table of references to all
the interrupt vector entry points can be used to dynamically
initialize the idt. The ramification for ramstage is that
exceptions are initialized later (lib/hardwaremain.c). Not
all stages initialize exceptions when this Kconfig variable
is selected, but bootblock for the C, stages using
assembly_entry.S, and of course ramstage do. Anything left
out just needs a call to exception_init() at the right
location.

BUG=b:72728953

Change-Id: I4146a040e5e43bed7ccc6cb0a7dc2271f1e7b7fa
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/25761
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>

src/arch/x86/Kconfig

@@ -302,3 +302,10 @@ config NUM_CAR_PAGE_TABLE_PAGES
 	depends on PAGING_IN_CACHE_AS_RAM
 	help
 	  The number of 4KiB pages that should be pre-allocated for page tables.
+
+# Provide the interrupt handlers to every stage. Not all
+# stages may take advantage.
+config IDT_IN_EVERY_STAGE
+	bool
+	default n
+	depends on ARCH_X86