cmake: Enable LTO for rpcs3_emu target

[Whatcookie]

In the past, enabling LTO across the entire program would cause some crashes. With GCC, LLVM would crash when compiling programs, and with clang, QT would crash. Let's try just enabling LTO for the rpcs3_emu target, instead of all the code of RPCS3.

Provides a small speedup (for linux users in titles not limited by SPU or PPU performance) and saves some space in the binary.

Before:

After:

Let's leave it as a draft until we can confirm the CI builds run crash free.

[cipherxof]

Clang CI build ran fine for me under linux.

[Romano44]

From the screenshots difference in that Ninja Gaiden game is about 22fps, which is ~15%! That is certainly more than a humble "small speedup".

[Whatcookie]

Both the GCC and Clang CI builds seem fine, so I unmarked it as draft.

[elad335]

Possibly limit it to RSX projects? what is the toll on compilation time?

[Whatcookie]

Possibly limit it to RSX projects? what is the toll on compilation time?

It adds less than a minute of time on my local builds. It's hard to tell what the impact is on the CI since the CI seems to have a large run to run variance to begin with.

It'd be nice to have LTO for more than just the RSX files since the PPU/SPU compilation is also noticeably faster.

[Yahfz]

With LTO enabled RSX performance is much better, allowing RDR to reach 90FPS+ for the first time