x86/cfi: Remove __noinitretpoline and __noretpoline

x86/cfi: Remove __noinitretpoline and __noretpoline

Commit 66f793099a63 ("x86/retpoline: Avoid retpolines for built-in __init
functions") disabled retpolines in __init sections (__noinitretpoline)
as a precaution against potential issues with retpolines in early boot,
but it has not been a problem in practice (i.e. see Clang below).

Commit 87358710c1fb ("x86/retpoline: Support retpoline builds with Clang")
narrowed this to only GCC, as Clang doesn't have per-function control
over retpoline emission. As such, Clang has been booting with retpolines
in __init since retpoline support was introduced.

Clang KCFI has been instrumenting __init since CFI was introduced.

With the introduction of KCFI for GCC, KCFI instrumentation with
retpolines disabled means that objtool does not construct .retpoline_sites
section entries for the non-retpoline KCFI calls. At boot, the KCFI
rehashing code, via __apply_fineibt(), misses all __init KCFI calls
(since they are not retpolines), resulting in immediate hash mismatches:
all preambles are rehashed (via .cfi_sites) and none of the __init call
sites are rehashed.

Remove __noinitretpoline since it provides no meaningful utility and
creates problems with CFI. Additionally remove __noretpoline since it
is now unused.

Alternatively, cfi_rand_callers() could walk the .kcfi_traps section which
is exactly the list of KCFI instrumentation sites. But it seems better to
have as few differences in common instruction sequences between compilers
as possible, so better to remove the special handling of retpolines in
__init for GCC.

Signed-off-by: Kees Cook <kees@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20250904034656.3670313-6-kees@kernel.org