libstdc++: Replace padding bits with bit-fields in __format::_Spec

This ensures that the unused bits will be zero-initialized reliably, and
so can be used later by assigning them values in formatter
specializations. For example, formatters for std::chrono will need to
use an extra bit for a boolean flag to optimize the conversions between
locale encodings and UTF-8.

Adding the 16-bit _M_reserved2 bit-field results in an increased size
for targets that use 1- or 2-byte alignment for all integral types, e.g.
cris-elf or m68k.  Placing that member before the _M_width member
adjusts the layout for all targets, but keeps all the bit-fields
together. We can't make that change once C++20 support is ABI stable and
non-experimental, so do it now before GCC 14 is released. The _M_fill
data member already change from char to char32_t in
r14-6991-g37a4c5c23a270c so _Spec is already incompatible with gcc-13
anyway.

libstdc++-v3/ChangeLog:

	* include/std/format (__format::_Spec::_M_reserved): Define new
	bit-field members to reserve padding bits for future extensions.