[gdb/build, c++20] Handle deprecated std::allocator::construct

When building gdb with -std=c++20, I run into:
...
gdbsupport/default-init-alloc.h:52:12: error: ‘construct’ has not been \
  declared in ‘class std::allocator<unsigned char>’
   52 |   using A::construct;
      |            ^~~~~~~~~
...

Indeed, std::allocator::construct has been deprecated in c++17 and removed in
c++20.

Fix this by using instead std::pmr::polymorphic_allocator for c++20.

Tested on x86_64-linux.

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