[5.3][CMake] fix runpath for ELF platforms

[finagolfin]

• Explanation

Remove the absolute path to the host toolchain's stdlib from libXCTest.so.

• Scope

I previously submitted a pull to do this for the main branch, #303, but after discussing it with some Swift devs, I'm now submitting it for the 5.3 branch without the $ORIGIN portion which we couldn't agree on. @compnerd seemed to agree that only the part removing the build toolchain's stdlib rpath should be done on the 5.3 branch to get this removed for the upcoming release.

• SR issue

Resolves point 2. in SR-1650

• Risk

None, this should be completely safe for the 5.3 branch, as it's just removing the build toolchain's stdlib rpath, which should be unused by this library.

• Testing

The other corelibs had similar pulls merged before the 5.3 release and have had no complaints.

• Reviewer

I discussed these rpath issues in detail across the entire toolchain with @gottesmm and @compnerd.

[compnerd]

@swift-ci please test

[compnerd]

CC: @shahmishal - what's the current strategy for PRs to 5.3? Is it okay to merge once it passes testing?

[compnerd]

@swift-ci please test Linux platform

[finagolfin]

Linux CI failure appears unrelated. Btw, since the similar SPM pull was held back for 5.3.1, I'm fine with waiting till then for this one too, as these are holes in the toolchain, not user executables.

[finagolfin]

@compnerd, the similar SPM pull was just merged, let me know if you can get this one in too.

[finagolfin]

@drexin, would be good to have this in for the next patch release, not sure what's up with the linux CI.

[finagolfin]

@spevans, would you run the CI on this too?

[spevans]

@swift-ci test

[gottesmm]

@briancroom can I get a +1 here on this?

[briancroom]

I'm not very familiar with the CMake configuration in here, but this seems reasonable, thanks!

[finagolfin]

@tomerd, just been waiting for a merge window to finally get this in.

[briancroom]

It looks like we can land this now. Thanks for pushing on this @buttaface!