tools/llvm: update to 18.1.7

tools/llvm: update to 18.1.7

- release notes:
  - https://discourse.llvm.org/t/llvm-16-0-0-release/69326
  - https://discourse.llvm.org/t/llvm-17-0-1-released/73549
  - https://discourse.llvm.org/t/llvm-18-1-0-released/77448
  - https://discourse.llvm.org/t/llvm-18-1-1-released/77540
  - https://discourse.llvm.org/t/18-1-2-released/77821
  - https://discourse.llvm.org/t/18-1-3-released/78136
  - https://discourse.llvm.org/t/18-1-4-released/78430
  - https://discourse.llvm.org/t/18-1-5-released/78740
  - https://discourse.llvm.org/t/18-1-6-released/79068
  - https://discourse.llvm.org/t/18-1-7-released/79433
- remove PKG_RELEASE (irrelevant to tools)
- set default target/triplet to "bpf"
- rearrange configuration options
- better control feature support and improve build reproducibility:
  disable auxiliary features that are enabled implicitly - their
  support is detected at build-time

Signed-off-by: Konstantin Demin <rockdrilla@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/15704
Signed-off-by: Robert Marko <robimarko@gmail.com>