update libunwind version to avoid aarch64-related issues

Summary:
X-link: facebookincubator/zstrong#1108

the current libunwind v1.8.1 has compiler issues with aarch64 (libunwind/libunwind#702). This has been fixed in more recent libunwind version.

Reviewed By: chadaustin

Differential Revision: D67800413

fbshipit-source-id: 6e924cf74909063be2319eb2263592f4c074e093

build/fbcode_builder/manifests/libunwind

@@ -9,10 +9,12 @@ libunwind
 [debs.not(distro=ubuntu)]
 libunwind-dev
 
+# The current libunwind v1.8.1 release has compiler issues with aarch64 (https://github.com/libunwind/libunwind/issues/702).
+# This more recent libunwind version (based on the latest commit, not a release version) got it fixed.
 [download]
-url = https://github.com/libunwind/libunwind/releases/download/v1.8.1/libunwind-1.8.1.tar.gz
-sha256 = ddf0e32dd5fafe5283198d37e4bf9decf7ba1770b6e7e006c33e6df79e6a6157
+url = https://github.com/libunwind/libunwind/archive/f081cf42917bdd5c428b77850b473f31f81767cf.tar.gz
+sha256 = 4ff5c335c02d225491d6c885db827fb5fa505fee4e68b4d7e866efc0087e7264
 
 [build]
 builder = autoconf
-subdir = libunwind-1.8.1
+subdir = libunwind-081cf42917bdd5c428b77850b473f31f81767cf