Web build/Emscripten needs optimization flags passed to the linker too (LINKFLAGS)

[akien-mga]

Tested versions

• Reproducible in 4.3.beta (ec02d40)

System information

Fedora Linux 40 (KDE Plasma) - Wayland - Vulkan (Forward+) - dedicated AMD Radeon RX 7600M XT (RADV NAVI33) - AMD Ryzen 7 7840HS w/ Radeon 780M Graphics (16 Threads)

Issue description

With @Faless we noticed that our optimization flags are passed to the linker currently for Emscripten Web builds, but Emscripten documents that they're needed there too:

https://emscripten.org/docs/tools_reference/emcc.html#emcc-os

Note that there's currently a regression so by default web builds use -O3 instead of -Os (#94087), but in both cases this should be added to the LINKFLAGS.

We should do a review of other emcc flags which need to be passed to both the compiler and the linker (which doesn't seem to be a requirement for GCC/Clang/MSVC, though that assumption might be worth revisiting too).

It does seem to make a significant difference, on a test "minimal" build with:

scons p=web target=template_release build_profile=../blank.build modules_enabled_by_default=no disable_3d=yes disable_advanced_gui=yes module_gdscript_enabled=yes production=yes verbose=yes

(which requires adding env.Append(LINKFLAGS=["-Wl,-u,htonl"]) for LTO to pass)
And with a manual fix for #94087, I get the following metrics:

-Os on CCFLAGS only:

• 16M for wasm
• 3.2M for zip

-Os on CCFLAGS and LINKFLAGS:

• 13M for wasm
• 3.0M for zip

Steps to reproduce

• scons p=web target=template_release verbose=yes
• See no -O[0-9s] option on the em++ linking call

Minimal reproduction project (MRP)

n/a