The UMA sampling profiler prefers unique symbol addresses, otherwise
disambiguation is fairly arbitrary (lexicographic).

To this end, introduce a dedicated symbol v8_code_start_for_profiler_
at a unique address (i.e. no other symbol is located at this address).

Bug: v8:6666
Change-Id: Iec13ccac64efc7ac3f63e29632ee8f6300bcb76b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2464926
Auto-Submit: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70556}