RFR(s): 8218147: make_walkable asserts on multiple calls

[Robbin Ehn]

Hi all, please review.

If a JavaThread in native both gets selected for java-stack sampling and a
handshake both VMThread and JFR sampler will call make_walkable. There is an
assert making sure we do not do this twice. Since we only store _last_Java_pc
from sp, we can allow it be executed multiple times for both aarch64/x64 which 
have the assert.

The asserts comes from:
8161598: Kitchensink fails: assert(nm->insts_contains(original_pc)) failed: 
original PC must be in nmethod/CompiledMethod

They seems not to be directly connected to the bug.

Issue:
https://bugs.openjdk.java.net/browse/JDK-8218147

Webrev:
http://cr.openjdk.java.net/~rehn/8218147/webrev/

Compiled aarch64, x64 passes t1-3.

Thanks, Robbin