Generator word now has two 16-bit components

Generator word now has two 16-bit components

The high 16-bits are a registered generator tool.
These are registered at
https://www.khronos.org/registry/spir-v/api/spir-v.xml

The low 16-bits are tool-specific.  It might be a version number,
for example, but is not constrained by the spec or by the registration
process.

The disassembler prints the tool name when we know it.
If we don't, print "Unknown" and then the numeric tool number
in parentheses.
In all cases, the disassembler prints lower 16-bit number on the
same line but after the tool name.

Also add newly registered generators:
  6: Khronos LLVM/SPIR-V Translator
  7: Khronos SPIR-V Tools Assembler