RFR(s): 8235410: Enable handshakes on Linux x86 (32-bit)

[Robbin Ehn]

Hi all, please review.

The flag ThreadLocalHandshakes is going to be obsolete in JDK 14.
So we change the (unchangeable) default for Linux x86 to on/true.

There is a follow-up which removes ThreadLocalHandshakes completely.
If the platform defines THREAD_LOCAL_POLL it will use handshakes.
When arm32 have implemented local polls, the plan is to remove the global poll
code paths.

Last time I checked with some of the affected parties there was no objections.

Built and sanity test.

Issue:
https://bugs.openjdk.java.net/browse/JDK-8235410

Code below.

Thanks, Robbin

diff -r 636d71e53732 src/hotspot/cpu/x86/globals_x86.hpp
--- a/src/hotspot/cpu/x86/globals_x86.hpp	Wed Dec 04 10:26:32 2019 +0100
+++ b/src/hotspot/cpu/x86/globals_x86.hpp	Thu Dec 05 14:13:57 2019 +0100
@@ -89,12 +89,7 @@

  define_pd_global(intx, InitArrayShortSize, 8*BytesPerLong);

-#if defined(_LP64) || defined(_WINDOWS)
  define_pd_global(bool, ThreadLocalHandshakes, true);
-#else
-// get_thread() is slow on linux 32 bit, therefore off by default
-define_pd_global(bool, ThreadLocalHandshakes, false);
-#endif

  #define ARCH_FLAGS(develop, \
                     product, \