[TLI][AArch64] Correct the mangled names for scalable vector functions.

When "s" is used as the ISA name, the compiler can make decisions
specific for SVE, based on VFABI specification.

This patch replaces _LLLVM_ ISA in TLI mappings with "s"
in the SVE specific (scalar to vector routine) mappings.

Author: JolantaJensen