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.

Only reset segments if changed (ia32)

Only force a reset of the segments (and a reload from the GDT) if the
segment selectors were changed by user-level.

RFC-3: Update context for x86 to use FS and GS.

TLS_BASE virtual register is replaced with FS_BASE and GS_BASE virtual
registers.

The FS_BASE and GS_BASE virtual registers are moved to the end of the
context so they need not be considered in the kernel exit and entry
implementation.

Removed tracking of ES, DS, FS, and GS segment selectors on kernel entry
and exit.

ES and DS are clobbered on kernel entry with the RPL 3 selector for a
DPL 3 linear data segment.

FS is clobbered on exit with the RPL 3 selector for the DPL 3 segment
with FS_BASE as the base. This is done on exit to reload the value from
the GDT.

GS is clobbered on exit with the RPL 3 selector for the DPL 3 segment
with GS_BASE as the base. This is done on exit to reload the value from
the GDT.

Kernel entry and exit code is refactored, simplified, and improved in
light of the above changes.

x64: update verified config to use fsgsbase instr

The verification platform for x64 relies on the fsgsbase instruction.

x86: FS/GS base MSRs

when FS/GS_BASE_MSR are used to set the base addreses,
user applications should not touch FS/GS regiters; so
the kernel should load proper selectors once, establishing
limits and other attributes for the segments.

x86: Move 32-bit specific files into 'mode' directories, and only build if IA32