[LibOS] Align address 'down' if MAP_FIXED is not set

Align address 'down' if MAP_FIXED is not set so that subsequent tests
for the alignment of the address are passing. This solves a memory
allocation issue of nodejs on ppc64.

LibOS/shim/src/sys/shim_mmap.c

@@ -62,6 +62,9 @@ void* shim_do_mmap(void* addr, size_t length, int prot, int flags, int fd, off_t
     struct shim_handle* hdl = NULL;
     long ret                = 0;
 
+    if (!(flags & MAP_FIXED) && addr)
+        addr = ALLOC_ALIGN_DOWN_PTR(addr);
+
     /*
      * According to the manpage, both addr and offset have to be page-aligned,
      * but not the length. mmap() will automatically round up the length.