replacing all ifndef with pargma once

All the kernel header files now use pargma once rather than the ifndef,
as the pre-processed C files do not change while header files
are protected with pargma once. This will also solve any naming issues
caused by ifndef.

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.

style: consistently attach return type

Add attach-return-type to astyle

x86: Separate header for TLB bitmap definitions

Having a separate header allows for them to be included into the platform hardware
definition without causing a circular include

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)

s/D61/DATA61/ in license headers for consistency

Refactor includes so trace point support builds

benchmark.h requires a definition of KS_LOG_PPTR, but its previous
placement in machine.h resulted in a circular include. This commit
factors out KS_LOG_PPTR and related definitions to a separate header,
creates the corresponding header for x86, and prevents circular
includes in the x86 builds

x86: Refactor tlb_bitmap to be mode generic

Refactors the TLB bitmap code to be generic across ia32 and x86-64.