[AArch64] Disable __ARM_FEATURE_SVE without ACLE.
The Arm C Language Extensions for SVE document specifies that
__ARM_FEATURE_SVE should be set when the compiler supports SVE and
implements all the extensions described in the document.
This is currently not yet the case, so the feature should be disabled
until the compiler can provide all the extensions as described.
Reviewers: c-rhodes, rengolin, rovka, ktkachov
Reviewed By: rengolin
Differential Revision: https://reviews.llvm.org/D65404