Package llvm12.0-doc: Information

- Add current mipsel patches
- Add riscv64 support:
  + fix clang-alt-triple.patch
  + disable lldb on riscv64 (no support yet)
  + backport .insn directive support for RISC-V from upstream
    (required to build recent rust).

- Included git-clang-format in clang-tools (closes: bug 40912).
  Split off LLVMgold.so to its own package.
  Added a requirement on llvmN-gold from llvmN-libs.

- Marked the package as built by ALT Linux Team (-DPACKAGE_VENDOR) so it's
  easier to distinguish from custom builds.
- If compiled with gcc, we now pass -ffat-lto-objects to it.
- Enabled -grecord-command-line by default (closes: bug 40628).
  Code generation is unaffected.