bash: disable bash malloc by default

Bash's malloc relies on sbrk which is implemented as a fail-only stub in
musl. Presently, it is disabled when configured for static
libs. Instead, default to using libc malloc.

Fixes:

  # bash
  bash: xmalloc: locale.c:81: cannot allocate 18 bytes (0 bytes allocated)

Signed-off-by: Daniel Sabogal <dsabogalcc@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>