Update to ARM gcc 15.2.Rel1

[dhalbert]

ARM gcc toolchain 14.3. Rel1 was released July 3, 2025: https://developer.arm.com/downloads/-/arm-gnu-toolchain-downloads

EDIT: ARM gcc toolchain 15.2 Rel1 was release December 17, 2025

[dhalbert]

I did some manual builds with 15.2Rel1. They are generally nicely smaller by a few hundred or thousand bytes, except that the nordic builds are about 6k larger. Need to investigate this.