[gdb] Fix hard-coded constants in buildsym_compunit::make_blockvector

[gdb] Fix hard-coded constants in buildsym_compunit::make_blockvector

I came across some code in buildsym_compunit::make_blockvector that uses
hardcoded constants 0 and 1:
...
      gdb_assert (blockvector->block (0)->is_global_block ());
      gdb_assert (blockvector->block (1)->is_static_block ());
...

Fix this by instead using the symbolic constants GLOBAL_BLOCK and
STATIC_BLOCK.

The same function has an odd-looking for loop that uses a hard-coded '1' to
skip the global block:
...
       /* The 'J > 1' here is so that we don't place the global block into
 	 the map.  For CU with gaps, the static block will reflect the
 	 gaps, while the global block will just reflect the full extent of
 	 the range.  */
      for (int j = num_blocks; j > 1; )
 	{
	  --j;
 	  struct block *b = blockvector->block (j);
...

Fix this by rewriting it into an ordinary descending for loop, and using
symbolic constant GLOBAL_BLOCK to avoid the global block:
...
      for (int j = num_blocks - 1; j > GLOBAL_BLOCK; --j)
 	{
 	  struct block *b = blockvector->block (j);
...

Approved-By: Tom Tromey <tom@tromey.com>