Preparation for merge of x86 and x86_64 kernel sources.

Since x86 and x86_64 share a lot of common code, x86_64 kernel sources/headers
are going to reside under headers/private/kernel/arch/x86 and
src/system/kernel/arch/x86 along with the existing x86 code. This commit
changes the build system to handle this. A new variable, TARGET_KERNEL_ARCH,
has been added. This is the name of the kernel/boot architecture directory
name, set to x86 on both x86 and x86_64. This is now used in all places where
TARGET_ARCH was used to get to kernel arch sources/headers (I've changed
everything necessary as far as I can tell). Kernel won't build for x86_64
at the moment as the sources have not been merged, loader does.

src/tools/gensyscalls/Jamfile

@@ -42,7 +42,7 @@ rule PreprocessSyscalls
 	local headers = [ on $(1) return $(SEARCH_SOURCE) $(SUBDIRHDRS) $(HDRS) ] ;
 	local sysHeaders =
 		$(TARGET_PRIVATE_SYSTEM_HEADERS)
-		[ ArchHeaders $(TARGET_ARCH) ]
+		[ ArchHeaders $(TARGET_KERNEL_ARCH) ]
 		[ on $(1) return $(SUBDIRSYSHDRS) $(SYSHDRS) ]
 		$(HAIKU_HDRS) ;