kconfig: Remove the architecture specific config for AutoFDO

kconfig: Remove the architecture specific config for AutoFDO

The CONFIG_AUTOFDO_CLANG option currently depends on
ARCH_SUPPORTS_AUTOFDO_CLANG, but this dependency seems unnecessary.

Remove ARCH_SUPPORTS_AUTOFDO_CLANG and allow users to control AutoFDO
builds solely through CONFIG_AUTOFDO_CLANG. This simplifies the kconfig
and avoids potential confusion.

Expand the AutoFDO documentation to include instructions for arm64.

Contributor acknowledgments:
  * SPE instructions: Daniel Hoekwater <hoekwater@google.com>
  * ETM instructions: Yabin Cui <yabinc@google.com>

Signed-off-by: Rong Xu <xur@google.com>
Suggested-by: Will Deacon <will@kernel.org>
Tested-by: Yabin Cui <yabinc@google.com>
Reviewed-by: Kees Cook <kees@kernel.org>
Link: https://patch.msgid.link/20260604195612.3757860-2-xur@google.com
Signed-off-by: Nathan Chancellor <nathan@kernel.org>