arm64: vdso: disable LTO

Due to a bug in clang, vdso fails to build when both LTO_CLANG and
CC_OPTIMIZE_FOR_SIZE are enabled:

  https://bugs.llvm.org/show_bug.cgi?id=32155



Disable LTO for vdso to work around the problem.

Bug: 62093296
Bug: 67506682
Change-Id: I1d0279535fd389db4c829e4556f9ef728f240a34
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>