Previously, we required baremetal runtimes to implement an undocumented
`__aarch64_sme_accessible` hook to check if SME is available (as
checking CPU features may vary across targets).
This allowed us to provide a generic `__arm_sme_state` implementation
but caused some friction for toolchains that depend on compiler-rt.
This patch instead removes the implementation of `__arm_sme_state` for
baremetal. This makes it the responsibility of the runtime (e.g. libc)
to provide this function for baremetal targets.
The requirements of this function are documented in the AAPCS64:
https://github.com/ARM-software/abi-aa/blob/main/aapcs64/aapcs64.rst#811__arm_sme_state
All other SME ABI rountines are still provided by compiler-rt.
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