x64: Fix Github issue #174

The PCID feature is the 17th bit in ECX when CPUID is called with
EAX=1. The constant for testing the feature should be 0x20000.

Signed-off-by: Yanyan Shen <yshen@cog.systems>

Convert license tags to SPDX identifiers

This commit also converts our own copyright headers to directly use
SPDX, but leaves all other copyright header intact, only adding the
SPDX ident. As far as possible this commit also merges multiple
Data61 copyright statements/headers into one for consistency.

x86_64: Update boot code documentation

x86_64: Add modular error handling (asm)

Adding a function to hang the system when necessary system conditions
are not met.

x86_64: Reduce scope of preprocessor condition

The fsgsbase_enable function can be completely removed when not
configured.

x86_64: Rework boot code to always use small pages

Previously, there was a check for whether or not huge pages are
supported by the processor before proceeding to set up the temporary
boot page table structures with the corresponding page size.

This is unnecessary as the page table structures are destroyed and
redefined again before entering user space.

The check for CONFIG_HUGE_PAGE is retained but not used for determining
the size of the boot page table structures. Instead, small pages are
always used as this is always supported by the architecture. This is a
beneficial change as it reduces the size and complexity of the untrusted
assembly boot code.

This change has the consequence of encouraging TLB misses during the
boot process but considering the size of the kernel and the lack of
running user space processes this is negligible.

x86_64: Fix incorrect assembly annotation

Fix trivial comment typos.

SMP: Introduce ENABLE_SMP_SUPPORT

- Make it more readable and less confusing compared to the 'CONFIG_MAX_NUM_NODES > 1' check

Fix licenses (the rest)

x86: Remove duplication of the multiboot header

x86: Explicitly define kernel stack size

This commit changes the previous hard coded 4K kernel stack size
to being a configurable power of 2 sized stack

Enable SYSCALL (SCE) after entering long mode

It appears to be somewhat more correct to flip SCE after entering
long mode, as per other operating systems (including Linux).

x86: Add BSS regions for BOOT and PHYS code

Uninitialized data structures in BOOT and PHYS code currently get
placed in sections that are allocated in the file of the final image.
Whilst these sections will get reclaimed during kernel boot, so no
runtime memory is being wasted, it results in kernel images that
are much larger to load and transport than necesary.

This change adds explicit BSS regions for both BOOT and PHYS code
and moves all appropriate data structures into them

x64: Add entry point for AP

x64: Add x86_64 support