build-system: Bags record their system and target.
* guix/build-system.scm (<bag>)[system, target]: New fields.
(make-bag): Add #:system parameter and pass it to LOWER.
* gnu/packages/bootstrap.scm (make-raw-bag): Initialize 'system' field.
* guix/build-system/cmake.scm (lower): Likewise.
* guix/build-system/perl.scm (lower): Likewise.
* guix/build-system/python.scm (lower): Likewise.
* guix/build-system/ruby.scm (lower): Likewise.
* guix/build-system/trivial.scm (lower): Likewise.
* guix/build-system/gnu.scm (lower): Initialize 'system' and 'target'
fields.
* guix/packages.scm (bag->derivation, bag->cross-derivation): New
procedures.
(package-derivation, package-cross-derivation): Use 'bag->derivation'.
* tests/packages.scm ("search paths"): Initialize 'system' and 'target'
fields.
("package->bag", "package->bag, cross-compilation", "bag->derivation",
"bag->derivation, cross-compilation"): New tests.
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