[docs] Prefer --gcc-install-dir= to deprecated GCC_INSTALL_PREFIX (#85458)

Setting GCC_INSTALL_PREFIX leads to a warning (#77537).

Link:
https://discourse.llvm.org/t/add-gcc-install-dir-deprecate-gcc-toolchain-and-remove-gcc-install-prefix/65091
Link:
https://discourse.llvm.org/t/correct-cmake-parameters-for-building-clang-and-lld-for-riscv/72833

Author: MaskRay

clang/www/get_started.html

@@ -86,13 +86,16 @@ <h3 id="buildNix">On Unix-like Systems</h3>
   </ul>
   </li>
 
-  <li>If you intend to use Clang's C++ support, you may need to tell it how
-      to find your C++ standard library headers. In general, Clang will detect
-      the best version of libstdc++ headers available and use them - it will
-      look both for system installations of libstdc++ as well as installations
-      adjacent to Clang itself. If your configuration fits neither of these
-      scenarios, you can use the <tt>-DGCC_INSTALL_PREFIX</tt> cmake option
-      to tell Clang where the gcc containing the desired libstdc++ is installed.
+  <li>On Linux, you may need GCC runtime libraries (e.g. <tt>crtbeginS.o,
+    libstdc++.so</tt>) and libstdc++ headers. In general, Clang will detect
+    well-known GCC installation paths matching the target triple (configured at
+    build time (see <tt>clang --version</tt>); overriden by
+    <tt>--target=</tt>) and use the largest version. If your configuration fits
+    none of the standard scenarios, you can set <tt>--gcc-install-dir=</tt> to
+    the GCC installation directory (something like
+    <tt>/usr/lib/gcc/$triple/$major</tt>). If your GCC installation is under
+    <tt>/usr/lib/gcc</tt> but uses a different triple, you can set
+    <tt>--gcc-triple=$triple</tt>.
   </li>
   <li>Try it out (assuming you add llvm/build/bin to your path):
   <ul>

openmp/docs/SupportAndFAQ.rst

@@ -279,11 +279,12 @@ Q: How to build an OpenMP offload capable compiler with an outdated host compile
 
 Enabling the OpenMP runtime will perform a two-stage build for you.
 If your host compiler is different from your system-wide compiler, you may need
-to set the CMake variable `GCC_INSTALL_PREFIX` so clang will be able to find the
-correct GCC toolchain in the second stage of the build.
+to set ``CMAKE_{C,CXX}_FLAGS`` like
+``--gcc-install-dir=/usr/lib/gcc/x86_64-linux-gnu/12`` so that clang will be
+able to find the correct GCC toolchain in the second stage of the build.
 
 For example, if your system-wide GCC installation is too old to build LLVM and
-you would like to use a newer GCC, set the CMake variable `GCC_INSTALL_PREFIX`
+you would like to use a newer GCC, set ``--gcc-install-dir=``
 to inform clang of the GCC installation you would like to use in the second stage.
 
 Q: How can I include OpenMP offloading support in my CMake project?