Found in testing against abseil. Two bugs were found:

1) SHIFT_AMOUNT in float_converter<long double> would sometimes be
   negative causing an underflow when passed as the amount to left shift
   by for BigInt.
2) is_lowest_block had an off-by-one because it was adding 1 to the
   block index.

Both are fixed and there are new tests to catch regressions.

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

The existing options for bin→dec float conversion are all based on the
Ryū algorithm, which generates 9 output digits at a time using a table
lookup. For users who can't afford the space cost of the table, the
table-lookup subroutine is replaced with one that computes the needed
table entry on demand, but the algorithm is otherwise unmodified.

The performance problem with computing table entries on demand is that
now you need to calculate a power of 10 for each 9 digits you output.
But if you're calculating a custom power of 10 anyway, it's easier to
just compute one, and multiply the _whole_ mantissa by it.

This patch adds a header file alongside `float_dec_converter.h`, which
replaces the whole Ryū system instead of just the table-lookup routine,
implementing this alternative simpler algorithm. The result is accurate
enough to satisfy (minimally) the accuracy demands of IEEE 754-2019 even
in 128-bit long double. The new float128 test cases demonstrate this by
testing the cases closest to the 39-digit rounding boundary.

In my tests of generating 39 output digits (the maximum number supported
by this algorithm) this code is also both faster and smaller than the
USE_DYADIC_FLOAT version of the existing Ryū code.

The sprintf tests have a macro named "ASSERT_STREQ_LEN" which was used
in about half of the tests. This patch moves all of the tests which can
to using that macro. This patch also enables long double tests for %e
and %g, since those never got finished. There's still some work to do
enabling long double testing for long doubles other than the intel 80
bit format, but that can land in a followup.

The `#ifdef LIBC_COPT_FLOAT_TO_STR_REDUCED_PRECISION` lines are for a
followup patch.

This patch adds the %m conversion to printf, which prints the
strerror(errno). Explanation of why is below, this patch also updates
the docs, tests, and build system to accomodate this.

The standard for syslog in posix specifies it uses the same format as
printf, but adds %m which prints the error message string for the
current value of errno. For ease of implementation, it's standard
practice for libc implementers to just add %m to printf instead of
creating a separate parser for syslog.

Fixes #98709.

This is a part of #97655.

Reverts llvm/llvm-project#98075

bots are broken

This is a part of #97655.