Modify run-pnacl-sz to pass in the correct assembler/disasembler flags
for ARM when not using the integrated assembler.

Model the "ret" pseudo instruction (special form of
"bx" inst). Separate from "bx" to allow epilogue
insertion to find the terminator.

Add a flag "--skip-unimplemented" to skip through all of the
"Not yet implemented" assertions, and use that in the test.

Set up a stack trace printer when ALLOW_DUMP so that the
UnimplementedError prints out some useful information of
*which* case is unimplemented.

Change the .type ...,@function from @function to %function.
ARM assembler seems to only like %function because
"@" is a comment character.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4076
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1136793002