README.md: Add information how to run GCC, Binutils, glibc tests

Let's document how to run all the test suites in order to increase the
number of people that will execute the tests.

Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>

README.md

@@ -210,6 +210,22 @@ Note:
 - spike only support rv64* bare-metal/elf toolchain.
 - gdb simulator only support bare-metal/elf toolchain.
 
+#### Testing GCC, Binutils, and glibc of a Linux toolchain
+
+The default Makefile target to run toolchain tests is `report`.
+This will run all tests of the GCC regression test suite.
+Alternatively, the following command can be used to do the same:
+
+    make check-gcc
+
+The following command can be used to run the Binutils tests:
+
+    make check-binutils
+
+The command below can be used to run the glibc tests:
+
+    make check-glibc-linux
+
 ### Development
 
 This section is only for developer or advanced user, or you want to build