lib9: make a p9frexp function wrapping system frexp

Under certain conditions it looks like frexp gets #defined
to something else on macOS during system headers,
which then breaks the declaration in libc.h.

include/libc.h

@@ -383,7 +383,7 @@ extern	int	encodefmt(Fmt*);
 extern	int	dirmodefmt(Fmt*);
 extern	int	exitcode(char*);
 extern	void	exits(char*);
-extern	double	frexp(double, int*);
+extern	double	p9frexp(double, int*);
 extern	ulong	getcallerpc(void*);
 #if defined(__GNUC__) || defined(__clang__)
 #define getcallerpc(x) ((ulong)__builtin_return_address(0))
@@ -436,6 +436,8 @@ extern	void	(*_unpin)(void);
 #define atoll		p9atoll
 #define encrypt		p9encrypt
 #define decrypt		p9decrypt
+#undef frexp
+#define frexp		p9frexp
 #define getenv		p9getenv
 #define	getwd		p9getwd
 #define	longjmp		p9longjmp

src/lib9/frexp.c

@@ -0,0 +1,9 @@
+#include <u.h>
+#define NOPLAN9DEFINES
+#include <libc.h>
+
+double
+p9frexp(double d, int *i)
+{
+	return frexp(d, i);
+}

src/lib9/mkfile

@@ -22,6 +22,7 @@ FMTOFILES=\
 	fmtstr.$O\
 	fmtvprint.$O\
 	fprint.$O\
+	frexp.$O\
 	nan64.$O\
 	print.$O\
 	runefmtstr.$O\