c-sky: enable fenv support in uClibc-ng config

target/csky/uclibc-ng.config

@@ -58,7 +58,7 @@ UCLIBC_HAS_FLOATS=y
 UCLIBC_HAS_FPU=y
 DO_C99_MATH=y
 # DO_XSI_MATH is not set
-# UCLIBC_HAS_FENV is not set
+UCLIBC_HAS_FENV=y
 UCLIBC_HAS_LONG_DOUBLE_MATH=y
 KERNEL_HEADERS=""
 HAVE_DOT_CONFIG=y