Pass '-fpie' to the clang linker driver on Linux, instead of 'pie'

[artemcm]

Resolves rdar://143793051

[artemcm]

@swift-ci test

[nkcsgexi]

Thank you!

[etcwilde]

I think the fix is fine. How haven't we hit this before though?
Also, maybe for embedded applications, do we have any support for emitting programs that aren't position independent?

[etcwilde]

CC @compnerd

[compnerd]

I'm confused by this change. The spelling is -pie for the linker driver invocation, but -fpie for the compiler invocation (unless you are on certain platforms, where the spelling is -fPIE) under certain circumstances.

-pie is the equivalent of -shared but for executables.

[artemcm]

Thanks for clearing that up, I had confused the two. @cachemeifyoucan pointed out that the warning we get appears to be due to the Clang driver emitting it for platforms where the toolchain default for executables is PIE.

[cachemeifyoucan]

I am actually unsure where the warning coming from before seeing the build log. I suggest it is better if you just pass -Xlinker -pie if you worried about unused argument warning.