Convert to SPDX license tags

Signed-off-by: Gerwin Klein <gerwin.klein@data61.csiro.au>

libsel4vm: Renamed 'reg_new_handler'

Renamed the x86 interface function 'reg_new_handler' to
'vm_reg_new_vmcall_handler'. This is to make the purpose of the
function clearer. This doesn't change any functionality.

trivial: Style changed files

libsel4vm: Removed uses of 'vmm' in x86 source

Updated x86 source such that the naming convention used is to
prefix/postfix with 'vm' rather than 'vmm'. This change makes
the naming convention consistent as this library now represents a
single 'vm' instance rather than a vmm.

libsel4vm: x86 vmcall interface to arch sub-dir

Moved the x86 vmcall interface into the architecture subdirectory.
This is an architecture specific interface we want to make
available to library users.