gdbsupport: remove remaining alloca uses

Remove the three remaining uses of alloca in gdbsupport.

I only built-tested the Windows-only portion in pathstuff.cc.

Change-Id: Ie588fa57f43de900d5f42e93a8875a7da462404b
Reviewed-By: Keith Seitz <keiths@redhat.com>
3 files changed