Rustbuild compiles proc macros with -Ztls-model=inital-exec when cross-compiling

[bjorn3]

Proc macros are dlopen'ed. -Ztls-model=initial-exec is not allowed for dlopen'ed dynamic libraries. Only for dynamic libraries declared directly or indirectly as DT_NEEDED of the main executable. While at least glibc reserves a bit of space just in case someone tries to dlopen a dynamic library compiled with the initial-exec tls model, this space is limited and running out of it will cause a failure to load.

cc https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/bypassing.20llvm-libunwind.20build.20on.20musl/near/268196922

[Mark-Simulacrum]

#100536 makes progress here for a subset of proc-macro dependencies; it's not an exhaustive list though and we don't have tests verifying we're doing the right thing here for now. It would be good to add those in the future.