Build lib/msun tests with compiler builtins disabled

This forces the compiler to emit calls to libm functions, instead of
possibly substituting pre-calculated results at compile time, which
should help to actually test those functions.

Reviewed by:	emaste, arichardson, ngie
Differential Revision: https://reviews.freebsd.org/D28577
MFC after:	3 days

lib/msun/tests/Makefile

@@ -19,6 +19,10 @@ CFLAGS+=	-I${TESTSRC:H}/libc/gen
 CFLAGS+=	-D__HAVE_LONG_DOUBLE
 .endif
 
+# Avoid builtins, to force the compiler to emit calls to the libm
+# functions, and not calculate any results in advance.
+CFLAGS+=        -fno-builtin
+
 NETBSD_ATF_TESTS_C=	acos_test
 NETBSD_ATF_TESTS_C+=	asin_test
 NETBSD_ATF_TESTS_C+=	atan_test