[Instrumented libraries] Move build files to xenial subdirectory

This is in preparation for adding a separate directory for focal.

In addition, the name of the prebuilt binaries changes from
msan-no-origins-xenial.tgz to msan-no-origins.tgz.  This is so that
projects like V8 do not have to manually update their DEPS entries
when auto-rolling //third_party/instrumented_libraries.

BUG=1260217
R=thestig
CQ_INCLUDE_TRYBOTS=luci.chromium.try:linux_chromium_msan_rel_ng

Change-Id: Id2eb94405f76e590cbc25b425c80eb08353d871f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3224627
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#932225}

DEPS

@@ -4092,7 +4092,7 @@ hooks = [
                 '--no_resume',
                 '--no_auth',
                 '--bucket', 'chromium-instrumented-libraries',
-                '-s', 'src/third_party/instrumented_libraries/binaries/msan-chained-origins-xenial.tgz.sha1',
+                '-s', 'src/third_party/instrumented_libraries/binaries/msan-chained-origins.tgz.sha1',
               ],
   },
   {
@@ -4104,7 +4104,7 @@ hooks = [
                 '--no_resume',
                 '--no_auth',
                 '--bucket', 'chromium-instrumented-libraries',
-                '-s', 'src/third_party/instrumented_libraries/binaries/msan-no-origins-xenial.tgz.sha1',
+                '-s', 'src/third_party/instrumented_libraries/binaries/msan-no-origins.tgz.sha1',
               ],
   },
   {

docs/linux/instrumented_libraries.md

@@ -99,7 +99,7 @@ Now we're ready to build the libraries. A clean build takes a little over 8
 minutes on a 72-thread machine.
 
 ```shell
-third_party/instrumented_libraries/scripts/build_and_package.py --parallel -j $(nproc) all
+third_party/instrumented_libraries/scripts/build_and_package.py --parallel -j $(nproc) all xenial
 ```
 
 ## Uploading the libraries
@@ -115,8 +115,7 @@ exit
 mv *.tgz third_party/instrumented_libraries/binaries
 
 # Upload.
-upload_to_google_storage.py -b chromium-instrumented-libraries third_party/instrumented_libraries/binaries/msan-chained-origins-xenial.tgz
-upload_to_google_storage.py -b chromium-instrumented-libraries third_party/instrumented_libraries/binaries/msan-no-origins-xenial.tgz
+upload_to_google_storage.py -b chromium-instrumented-libraries third_party/instrumented_libraries/binaries/msan*.tgz
 ```
 
 ## Testing and uploading a CL