msun: remove fabs from Symbol.map, and adjust comment

We have s_fabs.c, but fabs(3) is already provided by libc due to
historical reasons, so it is not compiled into libm. When the linker
does not use --undefined-version, this leads to a complaint about the
symbol being nonexistent, so remove it from Symbol.map.

While here, adjust the comment about some functions being supplied by
libc: while it is true that all these are indeed in libc, libm still
includes its own versions of frexp(3), isnan(3), isnanf(3), and
isnanl(3).

Reported by:	Steve Kargl <sgk@troutmask.apl.washington.edu>
MFC after:	3 days

(cherry picked from commit d04e03c19a8d7660ea36f62720eef9bc96b69889)

lib/msun/Makefile

@@ -140,7 +140,8 @@ COMMON_SRCS+=	catrig.c catrigf.c \
 	s_cproj.c s_cprojf.c s_creal.c s_crealf.c s_creall.c \
 	s_csinh.c s_csinhf.c s_ctanh.c s_ctanhf.c
 
-# BSD's C library supplies these functions:
+# BSD's C library supplies these functions (but note we still have s_frexp.c
+# and s_isnan.c enabled above, so they are duplicated):
 #COMMON_SRCS+=	s_fabs.c s_frexp.c s_isnan.c s_ldexp.c s_modf.c
 
 # Exclude the generic versions of what we provide in the MD area.

lib/msun/Symbol.map

@@ -88,7 +88,6 @@ FBSD_1.0 {
 	exp2f;
 	expm1;
 	expm1f;
-	fabs;
 	fabsf;
 	fabsl;
 	fdim;