Bertram Felgenhauer authored May 03, 2018 and Ömer Sinan Ağacan committed May 03, 2018

Before this patch, the pprUnwindwExpr function computed the length of
by the following assembly fragment:

	.uleb128 1f-.-1
	<expression data>
1:

That is, to compute the length, it takes the difference of the label 1
and the address of the .uleb128 directive, and subtracts 1.

In #15068 it was reported that `as` from binutils 4.30 has trouble with
evaluating the `.` part of the expression. However, there is actually a
problem with the expression, if the length of the data ever becomes
larger than 128: In that case, the .uleb128 directive will emit more
than 1 byte, and the computed length will be wrong.

The present patch changes the assembly fragment to use two labels,
which fixes both these problems.

	.uleb128 2f-1f
1:
	<expression data>
2:

Test Plan: validate

Reviewers: bgamari, osa1

Reviewed By: bgamari

Subscribers: thomie, carter

GHC Trac Issues: #15068

Differential Revision: https://phabricator.haskell.org/D4654