[5.8][build] Remove runpath from build host from shared ICU libraries on linux

[finagolfin]

Cherrypick of #64365

Explanation: I found this --enable-rpath flag in a hurry more than a year ago and suggested it before the branch in #40340, but was unaware it also adds an absolute runpath to the CI install directory:

> readelf -d swift-5.8-RELEASE-ubuntu20.04/usr/lib/swift/linux/libicu*so | ag "File:|runpath"
File: swift-5.8-RELEASE-ubuntu20.04/usr/lib/swift/linux/libicudataswift.so
 0x000000000000001d (RUNPATH)            Library runpath: [$ORIGIN:/home/build-user/build/buildbot_linux/swift-linux-x86_64/lib/swift/linux/x86_64]
File: swift-5.8-RELEASE-ubuntu20.04/usr/lib/swift/linux/libicui18nswift.so
 0x000000000000001d (RUNPATH)            Library runpath: [$ORIGIN:/home/build-user/build/buildbot_linux/swift-linux-x86_64/lib/swift/linux/x86_64]
File: swift-5.8-RELEASE-ubuntu20.04/usr/lib/swift/linux/libicuucswift.so
 0x000000000000001d (RUNPATH)            Library runpath: [$ORIGIN:/home/build-user/build/buildbot_linux/swift-linux-x86_64/lib/swift/linux/x86_64]

This is a deployment security risk, as these libraries are linked by Foundation, and thus is worth fixing in the current 5.8 release branch.

Scope: Only affects these libicu runpaths on ELF platforms like linux

Issue: None

Risk: low, as it only affects libicu runpaths for ELF platforms

Testing: Passed all CI, and we've had no problems since it was merged into trunk/5.9 a couple months ago.

Reviewer: @bnbarham

@tomerd, would be good to get this in before the next patch release.

[finagolfin]

@shahmishal, any chance we can get this small 5.8 tweak in before 5.8.1 is released?

[etcwilde]

@swift-ci please test

[etcwilde]

There may be someone abusing this who we'll break, but I'm okay not officially endorsing the rpath configuration.

[etcwilde]

@swift-ci Please Build Toolchain Linux Platform

[finagolfin]

All CI passed, plus I downloaded the linux toolchain built for this pull, checked that these CI runpaths were removed from the ICU libraries, and built and tested NIO with this modified 5.8 toolchain without a problem.

@DougGregor, this is ready to go in.