tcg: Increase minimum alignment from tcg_malloc to 8

For a 64-bit ILP32 host, aligning to sizeof(long) is not enough.
Guess the minimum for any host is 8, as that covers uint64_t.
Qemu doesn't use a host long double or host vectors, except in
extremely limited circumstances.

Fixes a bus error for a sparc v8plus host.

Signed-off-by: Richard Henderson <rth@twiddle.net>
