* 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
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