lang/pocl: Update 7.0 -> 7.1, improve port

Changelog:
This is mostly a bug-fix/maintenance release.

• Support for LLVM 21 for the CPU and LevelZero devices.
• Support for cl_khr_icd v2.0.0, cl_khr_spirv_queries and SPV_KHR_expect_assume.

https://newreleases.io/project/github/pocl/pocl/release/v7.1

Improve port:

• Switch to USES=llvm from hardcoded dependencies from llvm's files.
• Add options for build with all supported versions of the llvm.
• Cleanup RUN_DEPENDS=BUILD_DEPENDS antipattern.

PR: 290112 283227
Co-authored-by: Vladimir Druzenko <vvd@FreeBSD.org>