Implement aligned_alloc().

Implement aligned_alloc(), which was added in the C11 standard.  The
function is weakly specified to the point that a minimally compliant
implementation would be painful to use (size must be an integral
multiple of alignment!), which in practice makes posix_memalign() a
safer choice.

test/aligned_alloc.exp

@@ -0,0 +1,25 @@
+Test begin
+Alignment: 8
+Alignment: 16
+Alignment: 32
+Alignment: 64
+Alignment: 128
+Alignment: 256
+Alignment: 512
+Alignment: 1024
+Alignment: 2048
+Alignment: 4096
+Alignment: 8192
+Alignment: 16384
+Alignment: 32768
+Alignment: 65536
+Alignment: 131072
+Alignment: 262144
+Alignment: 524288
+Alignment: 1048576
+Alignment: 2097152
+Alignment: 4194304
+Alignment: 8388608
+Alignment: 16777216
+Alignment: 33554432
+Test end