Fix printf(3) output of long doubles on RISC-V

[mhorne]

Summary

When the RISC-V port was initially committed to FreeBSD, GCC would
generate 64-bit long doubles, and the definitions in _fpmath.h reflected
that. This was changed to 128-bit in GCC later that year [1], but the
definitions were never updated. This causes printf(3) and friends to
interpret only the low 64-bits of a long double in ldtoa, thereby
printing incorrect values.

Update the definitions now that both clang and GCC generate 128-bit long
doubles.

PR: 242067

[1] https://github.com/riscv/riscv-gcc/commit/54b21fc5ae83cefec44bc2caed4a8c664c274ba0

Test Plan

Verify that the test program from the PR gives the expected output for libc built with clang and gcc.

[emaste]

👍