[5.9][CMake] Don't add an extraneous rpath to the swift-frontend

[finagolfin]

Cherrypick of #64103

Explanation: The current 5.9 snapshot has an incorrect relative runpath added to the compiler on linux, the second one:

> readelf -d swift-5.9-DEVELOPMENT-SNAPSHOT-2023-05-22-a-ubuntu20.04/usr/bin/swift-frontend | ag runpath
 0x000000000000001d (RUNPATH)            Library runpath: [$ORIGIN/../lib/swift/linux:$ORIGIN/../../lib/swift/linux]

This pull fixes that, removing a mild security risk when running the compiler.

Scope: Only affects the compiler runpath on linux

Issue: None

Risk: low, as it only removes an unused runpath for the linux compiler

Testing: Passed all CI on trunk

Reviewer: @edymtt

[edymtt]

@swift-ci please test

[edymtt]

@swift-ci please build toolchain

[finagolfin]

Some problem on the macOS CI, toolchain build appears stuck.

[edymtt]

@swift-ci please test

[edymtt]

(sorry for the typo, misread the job that failed :( )

[edymtt]

@swift-ci please build toolchain macOS

[edymtt]

The generated Linux and macOS toolchains look good

$ readelf -d usr/bin/swift-frontend | grep -E "File:|runpath"
 0x000000000000001d (RUNPATH)            Library runpath: [$ORIGIN/../lib/swift/linux]

> otool -lv macos/Library/Developer/Toolchains/swift-PR-66121-708.xctoolchain/usr/bin/swift-frontend | grep -A2 RPATH
          cmd LC_RPATH
      cmdsize 32
         path /usr/lib/swift (offset 12)
--
          cmd LC_RPATH
      cmdsize 48
         path @executable_path/../lib/swift/host (offset 12)