Add an explanatory comment to lib/libc/gen/isnan.c about the fix to make static linking with libc and libm work.
Add an explanatory comment to lib/libc/gen/isnan.c about the fix to make
static linking with libc and libm work.

Requested by:	jilles
MFC after:	1 week
X-MFC-With:	242879

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase