Define LDBL_NBIT to be a mask indicating the position of the integer

bit in a long double.  For architectures that don't have such a bit,
LDBL_NBIT is 0.  This makes it possible to say `mantissa & ~LDBL_NBIT'
in places that previously used an #ifdef to select the right expression.
The optimizer should dispense with the extra arithmetic when LDBL_NBIT
is 0.

lib/libc/alpha/_fpmath.h

@@ -38,6 +38,7 @@ union IEEEl2bits {
 
 #define	mask_nbit_l(u)	((void)0)
 #define	LDBL_IMPLICIT_NBIT
+#define	LDBL_NBIT	0
 
 #define	LDBL_MANH_SIZE	20
 #define	LDBL_MANL_SIZE	32

lib/libc/amd64/_fpmath.h

@@ -38,7 +38,8 @@ union IEEEl2bits {
 	} bits;
 };
 
-#define	mask_nbit_l(u)	((u).bits.manh &= 0x7fffffff)
+#define	LDBL_NBIT	0x80000000
+#define	mask_nbit_l(u)	((u).bits.manh &= ~LDBL_NBIT)
 
 #define	LDBL_MANH_SIZE	32
 #define	LDBL_MANL_SIZE	32

lib/libc/arm/_fpmath.h

@@ -37,7 +37,8 @@ union IEEEl2bits {
 	} bits;
 };
 
-#define	mask_nbit_l(u)	((u).bits.manh &= 0x7fffffff)
+#define	LDBL_NBIT	0x80000000
+#define	mask_nbit_l(u)	((u).bits.manh &= ~LDBL_NBIT)
 
 #define	LDBL_MANH_SIZE	32
 #define	LDBL_MANL_SIZE	32

lib/libc/i386/_fpmath.h

@@ -37,7 +37,8 @@ union IEEEl2bits {
 	} bits;
 };
 
-#define	mask_nbit_l(u)	((u).bits.manh &= 0x7fffffff)
+#define	LDBL_NBIT	0x80000000
+#define	mask_nbit_l(u)	((u).bits.manh &= ~LDBL_NBIT)
 
 #define	LDBL_MANH_SIZE	32
 #define	LDBL_MANL_SIZE	32

lib/libc/powerpc/_fpmath.h

@@ -38,6 +38,7 @@ union IEEEl2bits {
 
 #define	mask_nbit_l(u)	((void)0)
 #define	LDBL_IMPLICIT_NBIT
+#define	LDBL_NBIT	0
 
 #define	LDBL_MANH_SIZE	20
 #define	LDBL_MANL_SIZE	32

lib/libc/sparc64/_fpmath.h

@@ -39,6 +39,7 @@ union IEEEl2bits {
 
 #define	mask_nbit_l(u)	((void)0)
 #define	LDBL_IMPLICIT_NBIT
+#define	LDBL_NBIT	0
 
 #define	LDBL_MANH_SIZE	48
 #define	LDBL_MANL_SIZE	64