[gdb/build, c++20] Fix build with gcc 10HEADmaster

With gcc 10 and -std=c++20, we run into the same problem as reported in commit
6feae66da1d ("[gdb/build, c++20] Handle deprecated std::allocator::construct").

The problem was fixed using:
...
-template<typename T, typename A = std::allocator<T>>
+template<typename T,
+         typename A
+#if __cplusplus >= 202002L
+         = std::pmr::polymorphic_allocator<T>
+#else
+         = std::allocator<T>
+#endif
+        >
...
but that doesn't work for gcc 10, because it defines __cplusplus differently:
...
 $ echo | g++-10 -E -dD -x c++ - -std=c++20 2>&1 | grep __cplusplus
 #define __cplusplus 201709L
 $ echo | g++-11 -E -dD -x c++ - -std=c++20 2>&1 | grep __cplusplus
 #define __cplusplus 202002L
...

Fix this by using the library feature test macro
__cpp_lib_polymorphic_allocator [1], which is undefined for c++17 and defined
for c++20:
...
 $ echo | g++-10 -E -dD -x c++ - -include memory_resource -std=c++17 2>&1 \
   | grep __cpp_lib_polymorphic_allocator
 $ echo | g++-10 -E -dD -x c++ - -include memory_resource -std=c++20 2>&1 \
   | grep __cpp_lib_polymorphic_allocator
 #define __cpp_lib_polymorphic_allocator 201902L
 $
...

A similar problem exists for commit 3173529d7de ("[gdb/guile, c++20] Work
around Werror=volatile in libguile.h").  Fix this by testing for 201709L
instead.

Tested on x86_64-linux, by building gdb with
{gcc 10, clang 17.0.6} x {-std=c++17, -std=c++20}.

PR build/32503
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32503

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

[1] https://en.cppreference.com/w/cpp/feature_test#cpp_lib_polymorphic_allocator

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