configure: Add --extra-filename flag

[brson]

This mixes in additional information into the hash that is
passed to -C extra-filename. It can be used to further distinguish
the standard libraries if they must be installed next to each
other.

Closes #29559

Frankly, I'm not sure if this solves a real problem. It's meant to help with side-by-side and overlapping installations where there are two sets of libs in /usr, but there are other potential issues there as well, including that some of our artifacts don't use this extra-filename munging, and it's not something our installers can support at all.

cc @jauhien Do you still think this helps the Gentoo case?

[brson]

r? @alexcrichton

[rust-highfive]

r? @nikomatsakis

(rust_highfive has picked a reviewer for you, use r? to override)

[jauhien]

@brson: thanks, this should help. At least it seems to cover Gentoo use case.

[alexcrichton]

@bors: r+ ce81f24

[bors]

⌛️ Testing commit ce81f24 with merge dedaebd...

[bors]

☀️ Test successful - auto-linux-32-nopt-t, auto-linux-32-opt, auto-linux-64-debug-opt, auto-linux-64-nopt-t, auto-linux-64-opt, auto-linux-64-x-android-t, auto-linux-cross-opt, auto-linux-musl-64-opt, auto-mac-32-opt, auto-mac-64-nopt-t, auto-mac-64-opt, auto-win-gnu-32-nopt-t, auto-win-gnu-32-opt, auto-win-gnu-64-nopt-t, auto-win-gnu-64-opt, auto-win-msvc-32-opt, auto-win-msvc-64-opt