haiku_loader: Fix wrong size of gBootGDT on x86_64.

The BOOT_GDT_SEGMENT_COUNT was based on USER_DATA_SEGMENT on both
x86 and x86_64. However, on x86_64 the order of the segments is
different, leading to a too small gBootGDT array. Move the define to
the arch specific headers so they can be setup correctly in either case.
Also add a STATIC_ASSERT() to check that the descriptors fit into the
array.

Pointed out by CID 1210898.

headers/private/kernel/arch/x86/32/descriptors.h

@@ -16,6 +16,8 @@
 #define USER_CODE_SEGMENT			3
 #define USER_DATA_SEGMENT			4
 
+#define BOOT_GDT_SEGMENT_COUNT		(USER_DATA_SEGMENT + 1)
+
 #define APM_CODE32_SEGMENT			5
 #define APM_CODE16_SEGMENT			6
 #define APM_DATA_SEGMENT			7

headers/private/kernel/arch/x86/64/descriptors.h

@@ -13,6 +13,8 @@
 #define USER_DATA_SEGMENT		3
 #define USER_CODE_SEGMENT		4
 
+#define BOOT_GDT_SEGMENT_COUNT	(USER_CODE_SEGMENT + 1)
+
 #define KERNEL_CODE_SELECTOR	((KERNEL_CODE_SEGMENT << 3) | DPL_KERNEL)
 #define KERNEL_DATA_SELECTOR	((KERNEL_DATA_SEGMENT << 3) | DPL_KERNEL)