Support achitecture-independent "Platform" field for Linux binaries

[jeroen]

As I understand the PACKAGES file on Linux is required to have a Platform: value to identify binary packages. (I don't remember why we cannot use the Built field). The Platform: can be e.g. x86_64-pc-linux-gnu or aarch64-unknown-linux-gnu:

• https://jeroen.r-universe.dev/bin/linux/noble-aarch64/4.5/src/contrib/PACKAGES
• https://jeroen.r-universe.dev/bin/linux/noble-x86_64/4.5/src/contrib/PACKAGES

However binary packages without compiled code are actually the same on both architectures. It would be nice to have a value of Platform: that allows for installation on any architecture.

Debian uses Architecture: all for this, for example: https://statmath.wu.ac.at/AASC/debian/binary-amd64/Packages