moarvm: fix build on powerpc64 / powerpc64le

moarvm: fix build on powerpc64 / powerpc64le

Build fails with:
 In file included from dyncall_callback.c:35:0:
dyncall_callback_ppc64.c: In function 'dcbNewCallback':
dyncall_callback_ppc64.c:42:13: warning: implicit declaration of function 'dcAllocWX' [-Wimplicit-function-declaration]
   int err = dcAllocWX(sizeof(DCCallback), (void**) &pcb);
             ^~~~~~~~~
dyncall_callback_ppc64.c: In function 'dcbFreeCallback':
dyncall_callback_ppc64.c:53:3: warning: implicit declaration of function 'dcFreeWX' [-Wimplicit-function-declaration]
   dcFreeWX(pcb, sizeof(DCCallback));
   ^~~~~~~~
dyncall_callback_ppc64.S: Assembler messages:
dyncall_callback_ppc64.S:180: Error: operand out of range (3 is not between 0 and 1)

So select BR2_PACKAGE_LIBFFI for BR2_powerpc64 and BR2_powerpc64le as it
is already done for MIPS

Fixes:
 - http://autobuild.buildroot.org/results/97b53a74d9847c07f26178daeb1daff3b6c24813
 - http://autobuild.buildroot.org/results/c35ac4bbc5fb04aabf5a719eddeedf55f7f1f4eb

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>