[Clang] Don't use crtbegin/crtend when building for musl.

[al45tair]

musl doesn't supply crtbegin/crtend objects, so we don't want to try to link them there.

rdar://123436174

[al45tair]

@swift-ci Please test

[TNorthover]

Would obviously need tests before upstreaming.

Other than that, if musl doesn't have crtbegin how does it get from entry to main properly? I know Darwin's dyld does it but hadn't heard of the Linux loader taking those duties.

[al45tair]

That's not what crtbegin.o and crtend.o are for. There's a useful page here that explains what all the object files are:

https://dev.gentoo.org/~vapier/crt.txt

It's crt1.o that's responsible for calling (eventually) main(). Musl is still using the "old-style" constructor/destructor system, hence it doesn't have crtbegin.o or crtend.o.

[TNorthover]

Fair enough. Think I was lumping all of those together in my mind.

[drodriguez]

  Clang :: Driver/ohos.c
  Clang :: Driver/ohos.cpp

fail after these changes. For example: https://ci.swift.org/job/pr-apple-llvm-project-llvm-linux/232/

I tested with these changes reverted and the tests pass. With these changes the tests fail.

[al45tair]

@drodriguez According to upstream LLVM, this is the wrong fix anyway, so let’s revert it.