RFR(M): 8204240: Extend MDO to allow more reasons to be recorded per bci

[Roland Westrelin]

http://cr.openjdk.java.net/~roland/8204240/webrev.00/

The number of traps that can be recorded per bci is currently limited to
8. This patch changes the layout of the DataLayout header so more traps
can be recorded per bci.

This change was part of 8203197: "C2: consider all paths in loop body
for loop predication" but I was asked to integrate separately. That
review thread has some discussion about that change.

Note this patch modifies platform specific code. I verified x86 (32 and
64 bits), aarch64. Sparc is ok AFAICT but I couldn't verify it. For PPC
and arm32 I will need someone to take a closer look.

Roland.