RFR(L): Remove global safepoint code

[Robbin Ehn]

Hi all, please build, test and review.

We removed (obsoleted) the flag ThreadLocalHandshakes in JDK 14.
Any platform defining THREAD_LOCAL_POLL can only use per thread poll.
Since all platforms now define THREAD_LOCAL_POLL the global poll code is 
dead.
Let's remove it!

Issue: https://bugs.openjdk.java.net/browse/JDK-8220051
Changeset: http://cr.openjdk.java.net/~rehn/8220051/v1/webrev/index.html

I have built fastdebug:
linux-aarch64
linux-arm32
linux-ppc64le
linux-s390x
linux-x86_64-zero
linux-x86_64 (no PCH)
linux-x86_64 (slowdebug) (no PCH)
linux-x86
osx-x86_64
windows-x86_64
solaris-sparcv9

Tested t1-5:
linux-x86_64
osx-x86_64
windows-x86_64
solaris-sparcv9

Manually:
linux-aarch64
linux-x86_64-zero

Any extra help building and testing is appreciated, especially on arm32, 
ppc64le, s390x.

Thanks, Robbin