This refactors the ThreadLocalTop into separate header and
implementation files, and moves it from the Isolate to the
IsolateData (with some tweaks to make the layout of the class
predictable). This has the advantage that all external references
referring to addresses in the ThreadLocalTop (like js_entry_sp,
c_function, c_entry_fp, etc.) need only a single memory access
to reach them. For example the CallApiCallback can now use

```
mov %rbp,0x8e40(%r13)
mov %rsi,0x8de0(%r13)
mov %rbx,0x8e50(%r13)
```

to setup the information about context, frame pointer, and C++
function pointer in the ThreadLocalTop instead of the previously
generated code

```
mov 0x2e28(%r13),%r10
mov %rbp,(%r10)
mov 0x2e38(%r13),%r10
mov %rsi,(%r10)
mov 0x2e30(%r13),%r10
mov %rbx,(%r10)
```

which always had to load the scratch register %r10 with the actual
address first. This has interesting performance impact. On the
test case mentioned in v8:8820 (with the `d8` patch applied), the
performance goes from

```
console.timeEnd: fnMono, 2290.012000
console.timeEnd: fnCall, 2604.954000
```

to

```
console.timeEnd: fnMono, 2062.743000
console.timeEnd: fnCall, 2477.556000
```

which is a pretty solid **10%** improvement for the monomorphic API
accessor case, and a **5%** improvement for calling into the API
accessor instead.

But there might as well be other places besides API callback calls
that will benefit from this change, which I haven't tested explicitly.

Although this change is supposed to be as minimal as possible without
any functional effects, some changes were necessary/logical. Eventually
we should reconsider changing the layout and the types for the fields
in the ThreadLocalTop to be more consistent with the other IsolateData
entities. But this can be done in separate follow-up CLs, as this will
be quite a bit of churn on the code base, depending on how we do that
exactly, and is orthogonal to this optimization.

Bug: v8:8820, v8:8848, chromium:913553
Change-Id: I4732c8e60231f0312eb7767358c48bae0338220d
Cq-Include-Trybots: luci.chromium.try:linux-blink-rel
Reviewed-on: https://chromium-review.googlesource.com/c/1474230Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Benedikt Meurer <bmeurer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59624}