LoongArch: BPF: Fix outdated tail call comments

LoongArch: BPF: Fix outdated tail call comments

The current LoongArch BPF JIT implementation hardcodes the number of
prologue instructions skipped during a tail call as a magic number '7'
in the jirl instruction. However, the accompanying comment explaining
this offset is completely outdated. It inaccurately states that only
a single TCC initialization instruction is bypassed, but in reality,
multiple setup slots are skipped, so fix these outdated comments in
__build_epilogue().

While at it, refine the comments in build_prologue() to describe the
skipped setup slots (RA saving, fentry nops, and the TCC register slot)
using proper dynamic tracing context. Also, remove the magic number '7'
by introducing descriptive macros to formally define the prologue layout
and make the tail call jump offset self-documenting.

Fixes: 61319d15a560 ("LoongArch: BPF: Adjust the jump offset of tail calls")
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>