cc_baremetal_defaults: Disable SVE

Prevent Clang from generating SVE instructions for compiler targets that
default to enabling the feature (such as ARMv9.2-A; see [1]) as the
baremetal environment might not have SVE available (either due to the HW
not supporting it or the SW not having enabled it).

[1]: https://reviews.llvm.org/D109517

Bug: 364383722
Test: lunch <...>  # Set TARGET_ARCH_VARIANT=armv9-2a
Test: m pvmfw      # Decompile & check for SVE instructions
Change-Id: Ic50d9dc45beb1d04d81e3accaf7fc4fc953ea703