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

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, despite the documented workaround. 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.

[1] 54b21fc5ae

PR:		242067
Reported by:	Dennis Clarke <dclarke@blastwave.org>
MFC after:	1 week
Differential Revision:	https://reviews.freebsd.org/D25420

lib/libc/riscv/_fpmath.h

@@ -46,19 +46,6 @@ union IEEEl2bits {
 #define	LDBL_IMPLICIT_NBIT
 #define	mask_nbit_l(u)	((void)0)
 
-#define	LDBL_MANH_SIZE	20
-#define	LDBL_MANL_SIZE	32
-
-#define	LDBL_TO_ARRAY32(u, a) do {			\
-	(a)[0] = (uint32_t)(u).bits.manl;		\
-	(a)[1] = (uint32_t)(u).bits.manh;		\
-} while(0)
-
-/*
- * TODO: Due to compiler problem we are temporary using
- * LDBL_PREC == 53. Use code below for LDBL_PREC == 113
- */
-#if 0
 #define	LDBL_MANH_SIZE	48
 #define	LDBL_MANL_SIZE	64
 
@@ -68,4 +55,3 @@ union IEEEl2bits {
 	(a)[2] = (uint32_t)(u).bits.manh;		\
 	(a)[3] = (uint32_t)((u).bits.manh >> 32);	\
 } while(0)
-#endif