boards/snps: Update documentation to reflect SDK path changes

All GNU toolchains have been moved underneath gnu/. Reflect this
change in the documentation about how to find the SDK version of gdb.

Signed-off-by: Keith Packard <keithp@keithp.com>

Author: keith-packard

boards/snps/em_starterkit/doc/index.rst

@@ -256,7 +256,7 @@ console, from the build directory containing the output :file:`zephyr.elf`.
 .. code-block:: console
 
    $ cd <my app>
-   $ $ZEPHYR_SDK_INSTALL_DIR/arc-zephyr-elf/bin/arc-zephyr-elf-gdb zephyr.elf
+   $ $ZEPHYR_SDK_INSTALL_DIR/gnu/arc-zephyr-elf/bin/arc-zephyr-elf-gdb zephyr.elf
    (gdb) target remote localhost:3333
    (gdb) load
    (gdb) b main

boards/snps/hsdk/doc/index.rst

@@ -467,7 +467,7 @@ console, from the build directory containing the output :file:`zephyr.elf`.
 .. code-block:: console
 
    $ cd <my app>
-   $ $ZEPHYR_SDK_INSTALL_DIR/arc-zephyr-elf/arc-zephyr-elf-gdb zephyr.elf
+   $ $ZEPHYR_SDK_INSTALL_DIR/gnu/arc-zephyr-elf/arc-zephyr-elf-gdb zephyr.elf
    (gdb) target remote localhost:3333
    (gdb) load
    (gdb) b main

boards/snps/hsdk4xd/doc/index.rst

@@ -463,7 +463,7 @@ console, from the build directory containing the output :file:`zephyr.elf`.
 .. code-block:: console
 
    $ cd <my app>
-   $ $ZEPHYR_SDK_INSTALL_DIR/arc-zephyr-elf/arc-zephyr-elf-gdb zephyr.elf
+   $ $ZEPHYR_SDK_INSTALL_DIR/gnu/arc-zephyr-elf/arc-zephyr-elf-gdb zephyr.elf
    (gdb) target remote localhost:3333
    (gdb) load
    (gdb) b main

boards/snps/iotdk/doc/index.rst

@@ -146,7 +146,7 @@ console, from the build directory containing the output :file:`zephyr.elf`.
 .. code-block:: console
 
    $ cd <my app>
-   $ $ZEPHYR_SDK_INSTALL_DIR/arc-zephyr-elf/bin/arc-zephyr-elf-gdb zephyr.elf
+   $ $ZEPHYR_SDK_INSTALL_DIR/gnu/arc-zephyr-elf/bin/arc-zephyr-elf-gdb zephyr.elf
    (gdb) target remote localhost:3333
    (gdb) load
    (gdb) b main