lldb: Fix usage of sve functions on arm64

Use correct internal sve functions for arm64.
Otherwise, when cross-compling lld for AArch64 there are build
errors like:
NativeRegisterContextLinux_arm64.cpp:936:11:
   error: use of undeclared identifier 'sve_vl_valid
NativeRegisterContextLinux_arm64.cpp:63:28:
    error: variable has incomplete type 'struct user_sve_header'

Reviewed By: omjavaid

Differential Revision: https://reviews.llvm.org/D148752