libstdc++: Fix P2510R3 "Formatting pointers" [PR110149]

libstdc++: Fix P2510R3 "Formatting pointers" [PR110149]

I had intended to support the P2510R3 proposal unconditionally in C++20
mode, but I left it half implemented. The parse function supported the
new extensions, but the format function didn't.

This adds the missing pieces, and makes it only enabled for C++26 and
non-strict modes.

libstdc++-v3/ChangeLog:

	PR libstdc++/110149
	* include/std/format (formatter<const void*, charT>::parse):
	Only alow 0 and P for C++26 and non-strict modes.
	(formatter<const void*, charT>::format): Use toupper for P
	type, and insert zero-fill characters for 0 option.
	* testsuite/std/format/functions/format.cc: Check pointer
	formatting. Only check P2510R3 extensions conditionally.
	* testsuite/std/format/parse_ctx.cc: Only check P2510R3
	extensions conditionally.