Fix `li-csv-to-ld-options` warning message

We used to check if the value of `/proc/sys/vm/mmap_min_addr` is at
least as high as the minimum segment of the input binary. If this is not
the case the linked program will segfault at run-time without much
explanation.

However, in truth, we need to be able to map also the page before the
lowest page the original binary mapped. The main reason for this is to
have space for the (outer) ELF header.

It turns out that on many distros the default minimum value is
`0x10000`, which happens to be exactly the same address at which ARM
binaries mmap their lowest page. This lead to no warning, but a segfault
at run-time.

The AWK script now checks for the correct value, and also suggests the
correct value.

In the future, we might want to create a new segment for the outer ELF
header and position it elsewhere in the address space.

li-csv-to-ld-options

@@ -24,10 +24,20 @@ END {
   #printf "-fuse-ld=gold "
   printf "-fuse-ld=bfd "
 
+  # Get the lowest page that can be allocate on this system
   getline mmap_min < "/proc/sys/vm/mmap_min_addr"
-  if (min < mmap_min) {
-    print "WARNING: The minimum address to map is too low: " min " (minimum allowed: " mmap_min "). Please run:" > "/dev/stderr";
-    print "echo " min " | sudo tee /proc/sys/vm/mmap_min_addr" > "/dev/stderr";
+
+  # We need to be able to allocate a page before the first one in the input
+  # program
+  lowest_page = min - 4096
+
+  # If such page cannot be allocate, the linked program will crash at run-time,
+  # therefore, warn the user about this
+  if (lowest_page < mmap_min) {
+    print "WARNING: The minimum address to map is too low: " lowest_page \
+          " (minimum allowed: " mmap_min "). Please run:" > "/dev/stderr";
+    print "echo " lowest_page \
+          " | sudo tee /proc/sys/vm/mmap_min_addr" > "/dev/stderr";
   }
 
   # If using gold omit these two lines