Update toolchain requirements and LLVM build docs (#49742)

Author: vchuravy

doc/src/devdocs/build/build.md

@@ -144,7 +144,7 @@ Notes for various architectures:
 Building Julia requires that the following software be installed:
 
 - **[GNU make]**                — building dependencies.
-- **[gcc & g++][gcc]** (>= 5.1) or **[Clang][clang]** (>= 3.5, >= 6.0 for Apple Clang) — compiling and linking C, C++.
+- **[gcc & g++][gcc]** (>= 7.1) or **[Clang][clang]** (>= 5.0, >= 9.3 for Apple Clang) — compiling and linking C, C++.
 - **[libatomic][gcc]**          — provided by **[gcc]** and needed to support atomic operations.
 - **[python]** (>=2.7)          — needed to build LLVM.
 - **[gfortran]**                — compiling and linking Fortran libraries.
@@ -169,7 +169,7 @@ repository) and then compiled from source the first time you run
 `make`. The specific version numbers of these libraries that Julia
 uses are listed in [`deps/$(libname).version`](https://github.com/JuliaLang/julia/blob/master/deps/):
 
-- **[LLVM]** (14.0 + [patches](https://github.com/JuliaLang/llvm-project)) — compiler infrastructure (see [note below](#llvm)).
+- **[LLVM]** (15.0 + [patches](https://github.com/JuliaLang/llvm-project/tree/julia-release/15.x)) — compiler infrastructure (see [note below](#llvm)).
 - **[FemtoLisp]**            — packaged with Julia source, and used to implement the compiler front-end.
 - **[libuv]**  (custom fork) — portable, high-performance event-based I/O library.
 - **[OpenLibm]**             — portable libm library containing elementary math functions.
@@ -238,7 +238,7 @@ The most complicated dependency is LLVM, for which we require additional patches
 For packaging Julia with LLVM, we recommend either:
  - bundling a Julia-only LLVM library inside the Julia package, or
  - adding the patches to the LLVM package of the distribution.
-   * A complete list of patches is available in `deps/llvm.mk`, and the patches themselves are in `deps/patches/`.
+   * A complete list of patches is available in on [Github](https://github.com/JuliaLang/llvm-project) see the `julia-release/15.x` branch.
    * The only Julia-specific patch is the lib renaming (`llvm-symver-jlprefix.patch`), which should _not_ be applied to a system LLVM.
    * The remaining patches are all upstream bug fixes, and have been contributed into upstream LLVM.