Introduce MODESEGMENT define; rename VISIBLE32 to VISIBLE32FLAT.

Prepare for support of segmented 32bit code.
Add new MODESEGMENT definition, and clarify existing 32bit mode defs.
diff --git a/src/smm.c b/src/smm.c
index 31e56d8..be2291b 100644
--- a/src/smm.c
+++ b/src/smm.c
@@ -11,7 +11,7 @@
 #include "ioport.h" // outb
 #include "pci_ids.h" // PCI_VENDOR_ID_INTEL
 
-ASM32(
+ASM32FLAT(
     ".global smm_relocation_start\n"
     ".global smm_relocation_end\n"
     ".global smm_code_start\n"