VM: Set breakpoints on x64 and arm64 without patching code.

VM: Set breakpoints on x64 and arm64 without patching code.

Instead of changing the generated code, change the target of calls
by changing the corresponding entry in the constant pool. Patchable
call sites can't share constant pool entries for now.
x64 and arm64 already had support for patching call targets in the
constant pool, but the debugger used to change the code instead.

This eliminates the fixed debugger stub entries in every constant pool.

[Florian Schneider, 2015-05-13 13:30:35]

fschneider@google.com changed reviewers:
+ hausner@google.com

[hausner, 2015-05-13 23:53:54]

LGTM. Extra points for removing more lines than adding new ones :)

[Florian Schneider, 2015-05-18 11:51:11]

On 2015/05/13 23:53:54, hausner wrote:
> LGTM. Extra points for removing more lines than adding new ones :)

Thanks! I plan to follow the same approach for ARM and MIPS in a separate CL so
that all platforms that use a constant pool are consistent.

[Florian Schneider, 2015-05-18 13:03:21]

Committed patchset #2 (id:20001) manually as
9191b76fd2d2567076c63066c0790e19522d98da (presubmit successful).