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.

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\