Return _methods_jmethod_ids field back in VMStructs

The fix for JDK-8313332 has removed InstanceKlass::_methods_jmethod_ids field from VMStructs:
https://github.com/openjdk/jdk/commit/21867c929a2f2c961148f2cd1e79d672ac278d27#diff-7d448441e80a0b784429d5d8aee343fcb131c224b8ec7bc70ea636f78d561ecd

This broke async-profiler, since the profiler needs this field to obtain jmethodID in some corner cases.

There was no actual need for removal, as the field is still there in InstanceKlass. So, I propose to return the field back to restore the broken functionality of async-profiler. This is a no risk change, because it only exports an offset of one field and does not affect the JVM otherwise.