The main thrust of this change is to generate a kernel that has the arm "marker" symbols stripped. Marker symbols start with $a, $d, or $t, and are emitted by the compiler to tell other toolchain components about the locations of data embedded in the instruction stream (literal-pool stuff). They are used for generating mixed-endian binaries (which we don't support). The linked kernel has approximately 21,000 such symbols in it, wasting space (500K in kernel.full, 190K in the final linked kernel), and sometimes obscuring function names in stack tracebacks.
This change also simplifies the way the kernel is linked. Instead of using sed to generate two different ldscript files to generate both an elf kernel and a binary (elf headers stripped) kernel, we now use a single ldscript that refers to a "text_start" symbol, and we provide the value for that symbol using --defsym on the linker command line.