Allow building the linker with -O0

The tlsdesc_resolver_dynamic function is only defined on arm64, but the
code was taking its address on all targets. (Apparently the optimizer was
removing the entire deferred_tlsdesc_relocs loop?)

Bug: none
Test: bionic unit tests
Test: add -O0 to cc_defaults in linker/Android.bp, device boots
Change-Id: I899c78018cbe82aeaca7856c9f7b04300c1d3196