debian-buster-build: add LLVMEmbeddedToolchainForArm
This is needed for simtrace2. As the bootloader goes beyond partition
size in modern gcc, we'll use clang instead.
Related: OS#5260
Related: simtrace2 I2aa2e20e75e334560dbe1f6db9fd1491873ff91f
Change-Id: Ib82a53fa7edc62d21e772efbb9b2c049d1b50c4d
