stdio/format: fix printing sign for long double

[rmikielis]

JIRA: RTOS-1368

Description

When down-converting from long double to double on RISCV, GCC is emitting __trunctfdf2() which truncates sign bit for special values (NaN, inf). This fix preservers sign bit before down-convertion, and adds it later when needed.

Motivation and Context

Fixes phoenix-rtos/phoenix-rtos-project#1007

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Chore (refactoring, style fixes, git/CI config, submodule management, no code logic changes)

How Has This Been Tested?

• Already covered by automatic testing.
• New test added: (add PR link here).
• Tested by hand on: (list targets here).

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing linter checks and tests passed.
• My changes generate no new compilation warnings for any of the targets.

Special treatment

• This PR needs additional PRs to work (list the PRs, preferably in merge-order).
• I will merge this PR by myself when appropriate.

[gemini-code-assist]

Code Review

This pull request updates stdio/format.c to preserve the sign bit when downconverting a long double to a double during floating-point formatting. It introduces an msb parameter to format_sprintfDouble to pass the sign bit. A review comment correctly points out that checking the sign bit with __builtin_signbit(doubleNumber) > 0 is unsafe, as the function is only guaranteed to return a non-zero value which could be negative on some platforms. It is recommended to use != 0 instead.

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[github-actions]

Unit Test Results

10 860 tests  ±0   10 190 ✅ ±0   53m 37s ⏱️ + 1m 45s
   670 suites ±0      670 💤 ±0 
     1 files   ±0        0 ❌ ±0 

Results for commit 3bcf8eb. ± Comparison against base commit 8ff4671.

♻️ This comment has been updated with latest results.