Never enable LTO for libunwind_llvm

Bug: http://b/78476183

Symbols from libunwind_llvm are supposed to have hidden visibility and
enforced with '-Wl,--exclude-libs'.  With LTO, the archive with these
symbols may not match the file to --exclude-libs, thereby causing them
to be exported.

Test: Verify that libhwbinder.so with LTO has hidden visiblity for
_Unwind_VRS_Set

Change-Id: I84292e97d6af42448165f515f2e11c6bc2ee9f63