v8_monolith always uses fvisibility=default, foiling LTO

[marcel]

I'm investigating binary size of compiled v8. I found that when using v8_monolith all source files are built with fvisibility=default. This is the correct option for embedders using dlopen (eg nodejs), but not for embedders using v8 without dynamically-loaded native code. 

When linking with LTO the public visibility means these symbols cannot be removed. In my application a one-line patch to v8.gni [https://github.com/laverdet/isolated-vm/blob/experimental/patches/10.symbols.patch], stripped binary size drops from 45M to 31M. After compression the comparison is 27M to 16M.

I tracked this decision back to:

https://github.com/v8/v8/commit/309944d0d08ed55823d78d883e70d27bd3b36cf7

I'm pretty surprised this condition has existed since 2018. I think the default should be hidden visibility for embedders, and those who use dlopen would add another flag to get those public symbols. Otherwise we're potentially doubling the binary size for embedders and the only recourse is patching v8's build.