RFR: 8343618: Stack smashing in awt_InputMethod.c on Linux s390x

[Zdenek Zambersky]

This fixes stack smashing issue in awt library on linux s390x. (jdk compiled with `-fstack-protector-strong`)

Fix is based on [patch](https://mail.openjdk.org/pipermail/awt-dev/2019-July/015337.html) submitted in JDK-8227919 review thread, rebased to master. They decided to go for Solaris only fix there, with [expected follow-up](https://mail.openjdk.org/pipermail/awt-dev/2019-July/015347.html) for linux/s390x. But that never happened.

I was not able to get response from original author for this issue, so I am creating PR myself.

**Testing:**
I tested jdk with and without this fix on linux/s390x (using Xvfb). It fixes the issue (reproducer for this bug no longer crashes the JVM). I have also tried to run `jdk_awt` tests, where lot of tests was also affected by this. With this patch amount of failures dropped by ~100. 

Without fix:

   TEST                                              TOTAL  PASS  FAIL ERROR  SKIP   
>> jtreg:test/jdk:jdk_awt                             2171  1063   415     6   687 <<


With fix

   TEST                                              TOTAL  PASS  FAIL ERROR  SKIP   
>> jtreg:test/jdk:jdk_awt                             2171  1162   316     6   687 <<


Many tests are no longer crashing the VM:

-java/awt/Choice/ChoiceKeyEventReaction/ChoiceKeyEventReaction.java                                                      Failed. Unexpected exit from test [exit code: 134]
+java/awt/Choice/ChoiceKeyEventReaction/ChoiceKeyEventReaction.java                                                      Passed. Execution successful

There seem to be few unstable tests in jdk_awt. (Ones that fail in one or another run, but seem unrelated to this fix)

I have not done any additional manual testing of XIM as did original author (in review thread mentioned higher), as I am not at all familiar with input method for Japanese (or other Asian characters), or how that supposed to work. (So I cannot verify that input of Asian characters works as expected on linux/s390x, but at least JVM no longer crashes.)

-------------

Commit messages:
 - Fix for stack smashing issue on linux s390x

Changes: https://git.openjdk.org/jdk/pull/24197/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24197&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8343618
  Stats: 20 lines in 1 file changed: 10 ins; 10 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/24197.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24197/head:pull/24197

PR: https://git.openjdk.org/jdk/pull/24197