This is the first step in preparation for:
https://discourse.llvm.org/t/rfc-make-clang-builtin-math-functions-constexpr-with-llvm-libc-to-support-c-23-constexpr-math-functions/86450

Summary:
We use the storage class for `long double` in the printing
implementations. We don't fully support the PPC double double type,
which that maps to, but we can stub out just the support needed for the
print interface to works. This required using the internal interface for
storage type, but it should be good enough.

Fixes: https://github.com/llvm/llvm-project/issues/136596

There is a problem with such unsigned comparison pattern:
```
if(unsigned_a - unsigned_b > 0) { /* only NOT go here when unsigned_a==unsigned_b */ }
```
When `unsigned_a` < `unsigned_b`, the result will still be `>0` due to
underflow.
This patch fixes two of the occurrences I found.
Also remove two redundant `if` where its condition is guaranteed by
outer `if`.

Commit 598e882ee88a1e3 turned `Writer` into a template, and updated most
of the call sites that use it. But not all. The alternative FP printf
system in `float_dec_converter_limited.h` wasn't updated, and neither
was `baremetal/printf.cpp` or `baremetal/vprintf.cpp`.

This patch updates `float_dec_converter_limited.h` in the same way that
the previous commit updated `float_dec_converter.h`: propagate the
templatedness through everything in the header, so that anything using a
`Writer` at all has a `write_mode` template parameter to pass on to it.

`printf.cpp` and `vprintf.cpp` are updated in the same way that the
previous commit updated `printf_core/vfprintf_internal.h`: the
`WriteBuffer` has parameter `WriteMode::FLUSH_TO_STREAM`, and `Writer`
picks it up implicitly from that.

Summary:
Currently we dispatch the writing mode off of a runtime enum passed in
by the constructor. This causes very unfortunate codegen for the GPU
targets where we get worst-case codegen because of the unused function
pointer for `sprintf`. Instead, this patch moves all of this to a
template so it can be masked out. This results in no dynamic stack and
uses 60 VGPRs instead of 117. It also compiles about 5x as fast.

Relates to
https://github.com/llvm/llvm-project/issues/119281#issuecomment-2699470459

This reverts commit 1e6e845d49a336e9da7ca6c576ec45c0b419b5f6 because it
changed the 1st parameter of adjust() to be unsigned, but libc itself
calls adjust() with a negative argument in align_backward() in
op_generic.h.

Relates to: #119281

These warnings all come up in code modified by one of my two recent
commits c06d0ff806b72b1 and b53da77c505a2d3, and all relate to implicit
integer type conversion. In a build of ours with strict compile options
two of them became errors. Even without that problem, it's worth fixing
them to reduce noise that might hide a more serious warning.