Implement mb2 stub support

Architecture/amd64/Bootstrap/Multiboot/2/GDT64.s

@@ -0,0 +1,63 @@
+/*
+   This file is part of Fennix Kernel.
+
+   Fennix Kernel is free software: you can redistribute it and/or
+   modify it under the terms of the GNU General Public License as
+   published by the Free Software Foundation, either version 3 of
+   the License, or (at your option) any later version.
+
+   Fennix Kernel is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with Fennix Kernel. If not, see <https://www.gnu.org/licenses/>.
+*/
+
+.intel_syntax noprefix
+
+.code64
+.section .bootstrap.data
+
+/* Access bits */
+.equ A, 1 << 0
+.equ RW, 1 << 1
+.equ DC, 1 << 2
+.equ E, 1 << 3
+.equ S, 1 << 4
+.equ DPL0, 0 << 5
+.equ DPL1, 1 << 5
+.equ P, 1 << 7
+
+/* Flags bits */
+.equ LONG_MODE, 1 << 5
+.equ SZ_32, 1 << 6
+.equ GRAN_4K, 1 << 7
+
+.global GDT64.Null
+.global GDT64.Code
+.global GDT64.Data
+.global GDT64.Tss
+.global GDT64.Ptr
+GDT64:
+.equ GDT64.Null, $ - GDT64
+	.quad 0
+.equ GDT64.Code, $ - GDT64
+	.long 0xFFFF
+	.byte 0
+	.byte P | S | E | RW
+	.byte GRAN_4K | LONG_MODE | 0xF
+	.byte 0
+.equ GDT64.Data, $ - GDT64
+	.long 0xFFFF
+	.byte 0
+	.byte P | S | RW
+	.byte GRAN_4K | SZ_32 | 0xF
+	.byte 0
+.equ GDT64.Tss, $ - GDT64
+	.long 0x00000068
+	.long 0x00CF8900
+GDT64.Ptr:
+	.word $ - GDT64 - 1
+	.quad GDT64