sdk: update to gcc 9.2

[bcressey]

Issue #, if available:
N/A

Description of changes:
The prior method of specifying the GCC version did not work. Now it does.

I also cleaned up ARG usage to fix the build on Docker 18.09, and changed the versioning scheme to match that of the admin and control containers.

Testing done:
Built x86_64 and aarch64 images with the updated SDK and tested them inside local VMs.

$ x86_64-thar-linux-gnu-gcc -v
Using built-in specs.
COLLECT_GCC=x86_64-thar-linux-gnu-gcc
COLLECT_LTO_WRAPPER=/usr/libexec/gcc/x86_64-thar-linux-gnu/9/lto-wrapper
Target: x86_64-thar-linux-gnu
Configured with: ./configure --prefix=/usr --libdir=/usr/lib --sysconfdir=/etc --enable-static --disable-libcc1 --with-sysroot=/x86_64-thar-linux-gnu/sys-root --with-build-sysroot=/home/builder/buildroot/output/x86_64-gnu/host/x86_64-thar-linux-gnu/sys-root --with-gcc-major-version-only --enable-default-pie --enable-default-ssp --target=x86_64-thar-linux-gnu --enable-__cxa_atexit --with-gnu-ld --disable-libssp --disable-multilib --disable-decimal-float --enable-linker-build-id --with-gmp=/home/builder/buildroot/output/x86_64-gnu/host --with-mpc=/home/builder/buildroot/output/x86_64-gnu/host --with-mpfr=/home/builder/buildroot/output/x86_64-gnu/host --with-pkgversion='Buildroot 2019.08.3' --with-bugurl=http://bugs.buildroot.net/ --enable-libquadmath --enable-tls --enable-plugins --enable-lto --enable-threads --with-isl=/home/builder/buildroot/output/x86_64-gnu/host --with-arch=nocona --enable-languages=c,c++ --with-build-time-tools=/home/builder/buildroot/output/x86_64-gnu/host/x86_64-thar-linux-gnu/bin --enable-shared=libgcc --disable-libgomp
Thread model: posix
gcc version 9.2.0 (Buildroot 2019.08.3) 

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