x86/tools: Rename test_get_len to insn_decoder_test

Rename test_get_len test command to insn_decoder_test
as it a more meaningful name. This also changes some
comments in related files.

Note that this also removes the paragraph about
writing to the Free Software Foundation's mailing
address.

Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/151153622537.22827.14928774603980883278.stgit@devbox


Signed-off-by: Ingo Molnar <mingo@kernel.org>