Mark Functions with __attribute__((hot)) and ((cold))

The Linux kernel annotates unlikely error paths with __attribute__((cold)) throughout its syscall implementations for the same reason glibc marks abort-path functions cold: moving rarely-executed code into the .text.unlikely section keeps the instruction cache packed with only the code that actually runs on the hot path. PGO effectively discovers and applies these same hot/cold splits automatically from real execution data, which is why this manual-annotation exercise is the mental model PGO is built on.