ANDROID: x86/vdso: disable LTO only for VDSO

Instead of disabling LTO for all the code in the directory, only
disable it for the VDSO itself.

Bug: 133186739
Bug: 145297900
Change-Id: I02e34b75e022982f9884e238aec89486890dd4bb
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>