prctl: rename branch landing pad implementation functions to be more explicit

prctl: rename branch landing pad implementation functions to be more explicit

Per Linus' comments about the unreadability of abbreviations such as
"indir_br_lp", rename the three prctl() implementation functions to be more
explicit.  This involves renaming "indir_br_lp_status" in the function
names to "branch_landing_pad_state".

While here, add _prctl_ into the function names, following the
speculation control prctl implementation functions.

Link: https://lore.kernel.org/linux-riscv/CAHk-=whhSLGZAx3N5jJpb4GLFDqH_QvS07D+6BnkPWmCEzTAgw@mail.gmail.com/
Cc: Deepak Gupta <debug@rivosinc.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mark Brown <broonie@kernel.org>
Signed-off-by: Paul Walmsley <pjw@kernel.org>