On ELF platforms, only add runpaths as needed

[finagolfin]

As long as I'm modifying the Python bootstrap script, clean up some incorrect checks of the host platform.system() and replace some unnecessary regexes.

It has always bothered me that since I suggested this hack of adding this second runpath for the Package libraries a couple years ago, all installed ELF binaries incorrectly have both runpaths:

> readelf -d swift-DEVELOPMENT-SNAPSHOT-2023-03-17-a-ubuntu20.04/usr/lib/swift/pm/*I/lib*so swift-DEVELOPMENT-SNAPSHOT-2023-03-17-a-ubuntu20.04/usr/bin/swift-package|ag "File:|runpath"
File: swift-DEVELOPMENT-SNAPSHOT-2023-03-17-a-ubuntu20.04/usr/lib/swift/pm/ManifestAPI/libPackageDescription.so
 0x000000000000001d (RUNPATH)            Library runpath: [$ORIGIN:$ORIGIN/../lib/swift/linux:$ORIGIN/../../linux]
File: swift-DEVELOPMENT-SNAPSHOT-2023-03-17-a-ubuntu20.04/usr/lib/swift/pm/PluginAPI/libPackagePlugin.so
 0x000000000000001d (RUNPATH)            Library runpath: [$ORIGIN:$ORIGIN/../lib/swift/linux:$ORIGIN/../../linux]
File: swift-DEVELOPMENT-SNAPSHOT-2023-03-17-a-ubuntu20.04/usr/bin/swift-package
 0x000000000000001d (RUNPATH)            Library runpath: [$ORIGIN:$ORIGIN/../lib/swift/linux:$ORIGIN/../../linux]

This adds the single correct relative runpath to each, in such a way so that it is only done when installing on the CI, as @ahoppen suggested in apple/sourcekit-lsp#715 and including his recommendation of removing regexes.

As noted there, this also speeds up the SPM build. I tested this natively on Android AArch64 without a problem.

I considered moving the macOS rpath flags to the package manifest too, then found that it may install all the SPM libraries (I'm assuming those flags are for macOS), so left that as a command-line flag applied to all libraries.

[neonichu]

@swift-ci please smoke test

[neonichu]

We probably also want a toolchain build for this.

[finagolfin]

Ran a toolchain build, downloaded the linux toolchain built, and checked that the rpaths are correct and that it works well.

The mac toolchain build failed only because the CI ran out of disk space:

86%: Link libSwiftPMDataModel.dylib (x86_64)
fatal error: lipo: can't write to output file: /Users/ec2-user/jenkins/workspace/swift-PR-toolchain-macos/branch-main/buildbot_osx/swiftpm-macosx-x86_64/apple/Products/Release/swift-bootstrap.dSYM/Contents/Resources/DWARF/swift-bootstrap.lipo
 (No space left on device)

[finagolfin]

Ping, ready to go in? I will submit for 5.9 next.