[sanitizer] Let glibc aarch64 use O(1) GetTls

The generic approach can still be used by musl and FreeBSD. Note: on glibc
2.31, TLS_PRE_TCB_SIZE is 0x700, larger than ThreadDescriptorSize() by 16, but
this is benign: as long as the range includes pthread::{specific_1stblock,specific}
pthread_setspecific will not cause false positives.

Note: the state before afec9538 underestimated
the TLS size a lot (nearly ThreadDescriptorSize() = 1776).
That may explain why afec9538 actually made some
tests pass.