[clang][SVE] Use __inline__ instead of inline in arm_sve.h

The inline keyword is not defined in the C89 standard, so source files
that include arm_sve.h will fail compilation if -std=c89 is specified.
For consistency with arm_neon.h, we should use __inline__ instead.

Reviewed By: paulwalker-arm

Differential Revision: https://reviews.llvm.org/D96852