gdb: Fix segfault with a big .dynamic section size

gdb: Fix segfault with a big .dynamic section size

Consider a binary with an erroneous size of the .dynamic section:

$ readelf -S a.out
...
  [24] .dynamic          DYNAMIC          0000000000004c20  00003c20
       000000fffffffa40  0000000000000010  WA       7     0     8
...

This binary causes a segfault in GDB.  GDB is trying to write the .dynamic
section into memory allocated on the stack with alloca().  However, the
allocation silently fails and the subsequent access to the memory is
causing the segfault. (On my node at least.)

Stack allocation is a bad idea for something of variable size that GDB has
no control over.  So I changed the code to heap allocation.

In addition, I changed the type of sect_size to the type that bfd actually
returns.

There should be no user visible change after this.

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