libc/common: Require at least double alignment for common malloc arena

arm, arc and riscv all have special cases for malloc arena alignment that
might be smaller than the minimum required for a C allocator. In
particular, the riscv value might actually be zero, which turns out to be
an invalid alignment value.

Make sure all of these have alignment that meets the C language
requirements for allocation alignment.

Signed-off-by: Keith Packard <keithp@keithp.com>