[bootstrap] add support for Ubuntu on RaspberryPi (#6647)

Summary of changes:
- Enable building OpenThread RCP image on the Raspberry Pi (aarch64)
  directly (using Ubuntu)
- Fix problem with not having access to the /opt folder as a user
diff --git a/script/bootstrap b/script/bootstrap
index 3af33a3..80df7e1 100755
--- a/script/bootstrap
+++ b/script/bootstrap
@@ -61,15 +61,16 @@
     echo 'Installing GNU Arm Embedded Toolchain...'
 
     PLATFORM=$(lsb_release -is)
+    ARCH=$(arch)
 
     if [ "$PLATFORM" = "Raspbian" ]; then
         sudo apt-get --no-install-recommends install -y binutils-arm-none-eabi gcc-arm-none-eabi gdb-arm-none-eabi libnewlib-arm-none-eabi libstdc++-arm-none-eabi-newlib
     elif [ "$PLATFORM" = "Ubuntu" ]; then
         sudo apt-get --no-install-recommends install -y ca-certificates wget
         (cd /tmp \
-            && wget -c https://developer.arm.com/-/media/Files/downloads/gnu-rm/9-2020q2/gcc-arm-none-eabi-9-2020-q2-update-x86_64-linux.tar.bz2 \
-            && tar xjf gcc-arm-none-eabi-9-2020-q2-update-x86_64-linux.tar.bz2 -C /opt \
-            && rm gcc-arm-none-eabi-9-2020-q2-update-x86_64-linux.tar.bz2 \
+            && wget -c https://developer.arm.com/-/media/Files/downloads/gnu-rm/9-2020q2/gcc-arm-none-eabi-9-2020-q2-update-"$ARCH"-linux.tar.bz2 \
+            && sudo tar xjf gcc-arm-none-eabi-9-2020-q2-update-"$ARCH"-linux.tar.bz2 -C /opt \
+            && rm gcc-arm-none-eabi-9-2020-q2-update-"$ARCH"-linux.tar.bz2 \
             && sudo ln -s -f /opt/gcc-arm-none-eabi-9-2020-q2-update/bin/* /usr/local/bin/.)
     fi