tools/llvm: update to 20.1.8

tools/llvm: update to 20.1.8

- release notes:
  - https://discourse.llvm.org/t/18-1-8-released/79725
  - https://discourse.llvm.org/t/llvm-19-1-0-released/81285
  - https://discourse.llvm.org/t/llvm-19-1-1-released/82321
  - https://discourse.llvm.org/t/llvm-19-1-2-released/82550
  - https://discourse.llvm.org/t/llvm-19-1-3-relased/82829
  - https://discourse.llvm.org/t/llvm-19-1-4-released/83230
  - https://discourse.llvm.org/t/llvm-19-1-5-released/83455
  - https://discourse.llvm.org/t/llvm-19-1-6-released/83723
  - https://discourse.llvm.org/t/llvm-19-1-7-released/84062
  - https://discourse.llvm.org/t/llvm-20-1-0-released/85122
  - https://discourse.llvm.org/t/llvm-20-1-1-released/85337
  - https://discourse.llvm.org/t/llvm-20-1-2-released/85654
  - https://discourse.llvm.org/t/llvm-20-1-3-released/85874
  - https://discourse.llvm.org/t/llvm-20-1-4-released/86147
  - https://discourse.llvm.org/t/llvm-20-1-5-released/86371
  - https://discourse.llvm.org/t/llvm-20-1-6-released/86598
  - https://discourse.llvm.org/t/llvm-20-1-7-released/86855
  - https://discourse.llvm.org/t/llvm-20-1-8-released/87259
- remove "100-gcc15.patch" (not needed anymore)
- refresh configuration options

Signed-off-by: Konstantin Demin <rockdrilla@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/19417
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>