RFR (S): 7129724: Fix location of core file in dump message on macosx

[Mikael Vidstedt]

If HotSpot crashes the dump message prints the default location of the 
generated core file. On other "posix" like OSes the path is normally 
<cwd>/core or core.<pid>, but on macosx the cores always end up in 
/cores/core.<pid>. This change aims to reflect that. Verified manually 
on Linux and macosx. Passes JPRT.

Webrev: http://cr.openjdk.java.net/~mikael/7129724/webrev.00

Cheers,
Mikael