malloc: Use __getrandom_nocancel during tcache initiailization

Cancellation currently cannot happen at this point because dlopen
as used by the unwind link always performs additional allocations
for libgcc_s.so.1, even if it has been loaded already as a dependency
of the main executable.  But it seems prudent not to rely on this
quirk.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>