[8u] make java_lang_String::hash_code consistent across platforms

java_lang_String::hash_code produces inconsistent results on different platforms, when s is char*. This is because on some platforms char is signed, while on others unsigned (resulting in char to be either zero-extended or sign-extended, when cast to unsigned int). This causes 1 tier1 test failure on aarch64.

Details:
This was discovered by examining one failing test (from tier1) present on aarch64 builds:
test/serviceability/sa/jmap-hashcode/Test8028623.java
Test was introduced by JDK-8028623. However fix done there does not work on aarch64. Code was later fixed (newer jdks) in hotspot part of JDK-8141132 (JEP 254: Compact Strings).

Fix:
Fixed by backporting very small portion of JDK-8141132.

Testing:
tier1 (x86, x86_64, aarch64): OK (tested by GH and on aarch64 machine)