[SYCL][Doc] Update DPC++ prerequisites (#10509)

We used to copy LLVM's requirements for the host compiler in our Get
Started Guide, but it's easy to miss the updates in LLVM project
requirements. This patch removes the duplication and links LLVM's
up-to-date documentation.

Author: bader

sycl/doc/GetStartedGuide.md

@@ -44,10 +44,7 @@ and a wide range of compute accelerators such as GPU and FPGA.
 * `ninja` -
 [Download](https://github.com/ninja-build/ninja/wiki/Pre-built-Ninja-packages)
 * C++ compiler
-  * Linux: `GCC` version 7.1.0 or later (including libstdc++) -
-    [Download](https://gcc.gnu.org/install/)
-  * Windows: `Visual Studio` version 15.7 preview 4 or later -
-    [Download](https://visualstudio.microsoft.com/downloads/)
+  * See LLVM's [host compiler toolchain requirements](../../llvm/docs/GettingStarted.rst#host-c-toolchain-both-compiler-and-standard-library)
 
 Alternatively, you can use a Docker image that has everything you need for building
 pre-installed: