[gdb/build, c++20] Stop using deprecated is_pod

When building gdb with clang 15 and -std=c++20, I run into:
...
gdbsupport/poison.h:52:11: error: 'is_pod<timeval>' is deprecated: use \
  is_standard_layout && is_trivial instead [-Werror,-Wdeprecated-declarations]
            std::is_pod<T>>
                 ^
...

Fix this by following the suggestion.

Likewise in gdb/unittests/ptid-selftests.c.

Tested on x86_64-linux.