- add gcc-15.2.0 to steps-guix manifest after binutils-2.41
- keep full gcc-15.2.0 pass1 src_prepare cleanup/regeneration flow
- switch configure/build/install to kernel toolchain bootstrap mode:
--without-headers, --enable-multilib, all-gcc, all-target-libgcc
- install into /kernel-toolchain and prioritize /kernel-toolchain/bin in PATH
- add missing gcc distfiles entry for SARIF spec
- include decDPD helper files used during src_prepare
