Flat address x86_64 port of Nuttx (#411)

* arch: x86_64: Pour-in the x86_64 code from cRTOS repository, excluding modifications of NuttX kernel, jailhouse support and linux compatibility layer

* arch: x86_64: Refactor x86_64 loading procedure for better comprehension and included support for multiboot2

* arch: x86_64: Locate the kernel at 4GB~ and modify the page table initializing procedure accordingly

* arch: x86_64: Implemented kconfig option for various x86_64 capabilities, dynamic probe and check capability on lowsetup before enabling

* arch: x86_64: inte64_check_capability: Use Marco to prettify the capability checking procedure

* arch: x86_64: intel64_timerisr.c: Refactor with new frequency calibrating method

* arch: x86_64: Fix C alias of page table and GDT/IST

* arch: x86_64: Reload GTDR with GDT in high address in up_lowsetup

* arch: x86_64: Consolidate MSR definition in arch/arch.h

* arch: x86_64: Edit the way of handling GDT/IST in C into structures

* arch: x86_64: Correct the starting point of isr/irq stack

* arch: x86_64: Update up_initialize.c with the new initializing procedure

* arch: x86_64: up_map_region now take flags instead of assuming WR/PRESENT

* arch: x86_64: Overhual of interrupt initialization procedure

* arch: x86_64: Properly configure the heap to be memory as [_ebss, end of memory]

* arch: x86_64: Try to probe the TSC frequency, fall-back to user specified frequency on failure

* arch: x86_64: Remove debug printing during restore_aux, causing infinite CTX bug

* arch: x86_64: for X86 16500 serial interrupt to work, OUT2 of MCR must be 1. Make it stuck at 1 after boot

* arch: x86_64: Correctly apply license header, comment and format code

* arch: x86_64: properly send a SIGFPE on floating point error

* arch: x86_64: Remove unused variable in up_restore_auxstate

* arch: x86_64: properly trash the processor with an infinite loop

* arch: x86_64: Fix typo in ISR handler causing ISR not handled

* arch: x86_64: Fix possibile race conditions with scheduler debug option on in signal handling path

* arch: x86_64: Fix typo in MSR_X2APIC_LVTT_TSC_DEADLINE

* arch: x86_64: Migrate tickless implementation to the new MSR naming and frequency calibration method

* board: x86_64: qemu: Add guard to exclude up_netinitialize when compiling without net support

* arch: x86_64: update defconfigs

* arch: x86_64: rename qemu as qemu-intel64

* arch: x86_64: update Board readme

arch/Kconfig

@@ -79,10 +79,15 @@ config ARCH_SIM
 	---help---
 		Linux/Cygwin user-mode simulation.
 
-config ARCH_X86
+config arch_x86
 	bool "x86"
 	---help---
-		Intel x86 architectures.
+		intel x86 architectures.
+
+config ARCH_X86_64
+	bool "x86_64"
+	---help---
+		x86-64 architectures.
 
 config ARCH_XTENSA
 	bool "Xtensa"
@@ -136,6 +141,7 @@ source arch/renesas/Kconfig
 source arch/risc-v/Kconfig
 source arch/sim/Kconfig
 source arch/x86/Kconfig
+source arch/x86_64/Kconfig
 source arch/xtensa/Kconfig
 source arch/z16/Kconfig
 source arch/z80/Kconfig