Fixes a couple of bugs found when building. The PR to enable the headers
can be found here: #144114.

- math.yaml: float128 guard
- wchar.yaml: __restrict keyword order

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

Prevously, if INT_MIN was passed as a wildcard width to a printf
conversion the parser would attempt to negate it to get the positive
width (and set the left justify flag), but it would underflow and the
width would be treated as 0. This patch corrects the issue by instead
treating a width of INT_MIN as identical to -INT_MAX.

Also includes docs changes to explain this behavior and adding b to the
list of int conversions.

Resolves #81685. This adds support for wN and wfN length modifiers in
fprintf.

Reference: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2612.pdf.

Fixes https://github.com/llvm/llvm-project/issues/80727.

This is step 4 of
https://discourse.llvm.org/t/rfc-customizable-namespace-to-allow-testing-the-libc-when-the-system-libc-is-also-llvms-libc/73079

Summary:
The parser class for stdio currently accepts different argument
providers. In-tree this is only used for a fuzzer test, however, the
proposed implementation of the GPU handling of printf / scanf will
require custom argument handlers. This makes the current approach of
using a preprocessor macro messier. This path proposed folding this
logic into a template instantiation. The downside to this is that
because the implementation of the parser class is placed into an
implementation file we need to manually instantiate the needed templates
which will slightly bloat binary size. Alternatively we could remove the
implementation file, or key off of the `libc` external packaging macro
so it is not present in the installed version.