Fix finding entry point unwind record for a method with fragmented un… (#22202)

Fix finding entry point unwind record for a method with fragmented un… (#22202)

On Arm and Arm64 unwind records can only cover a limited range of code
(512K and 1MB respectively). So for methods larger than this the jit
will emit multiple "fragment" unwind records to cover the full method code
range. Only the first of these describes the behavior of the method prolog.

When mapping an offset back to a method's entry point unwind, make sure to
find this "root" unwind record instead of one of the internal fragments.

Fixes #19209.