newlib: Fix memory leak regarding gdtoa-based _ldtoa_r().

newlib: Fix memory leak regarding gdtoa-based _ldtoa_r().

After the commit a4705d387f78, printf() for floating-point values
causes a memory leak. The legacy _ldtoa_r() assumed the char pointer
returned will be free'ed by Bfree(). However, gdtoa-based _ldtoa_r()
returns the pointer returned by gdtoa() which should be free'ed by
freedtoa(). Due to this issue, the caller of _ldtoa_r() fails to free
the allocated char buffer. This is the cause of the said memory leak.
https://cygwin.com/pipermail/cygwin/2023-July/254054.html

This patch makes rv_alloc()/freedtoa() allocate/free the buffer in
a compatible way with legacy _ldtoa_r().

Fixes: a4705d387f78 ("ldtoa: Import gdtoa from OpenBSD.")
Reported-by: natan_b <natan_b@libero.it>
Reviewed-by: Corinna Vinschen <corinna@vinschen.de>
Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>