RFR (S): 8191681: Unsafe accesses should use field_offset_to_byte_offset()

[Erik Österlund]

Hi,

Before the Access API, there was a no-op conversion function in 
unsafe.cpp for getting the byte offset of a field offset from unsafe as 
a jlong. Now, the field offsets *are* byte offsets, so the conversion 
did not do anything. But I guess it could be useful nevertheless for 
abstraction purposes.

In this patch I have a new function called field_offset_to_access_offset 
that gets the offset to be used by Access (ptrdiff_t) from a field 
offset, and uses that for the access calls.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8191681

Webrev:
http://cr.openjdk.java.net/~eosterlund/8191681/webrev.00/

Thanks,
/Erik