use linker optimizations

[thestinger]

Among other possibilities, GNU's ld (ld.bfd) and ld.gold have an -O switch to perform various optimizations. It makes them spend more time optimizing lookup table performance/size. Rust has long mangled symbol names so these optimizations are going to be important once there are large projects with lots of symbols. We probably also want to use --as-needed and perhaps --gc-sections.

Sadly Apple's ld is pretty crippled and lacks the -O switch, and I'm unsure about how portable these flags are overall. Although this issue would be bypassed by eventually using LLVM's lld linker.

[catamorphism]

(bug triage) Good idea, but seems like just-a-bug to me.

[emberian]

I'll take a poke at it this week.

[sanxiyn]

This seems to have landed. What happened to Apple's ld situation?

[thestinger]

@sanxiyn: it's only enabled on Linux at the moment, and we could likely flip it on for Windows too since we use MinGW and will be able to pass the same flags to lld

[alexcrichton]

Closed on linux by #10620, I'm not sure if this exists much more on other platforms, and most of the concerns of this are probably going to get subsumed by #10741. Closing for now (but feel free to reopen if anyone thinks otherwise!)