uClibc-ng: enable FENV support for x86-64

target/x86_64/uclibc-ng.config

@@ -52,7 +52,7 @@ UCLIBC_HAS_FLOATS=y
 UCLIBC_HAS_FPU=y
 DO_C99_MATH=y
 DO_XSI_MATH=y
-# UCLIBC_HAS_FENV is not set
+UCLIBC_HAS_FENV=y 
 UCLIBC_HAS_LONG_DOUBLE_MATH=y
 KERNEL_HEADERS=""
 HAVE_DOT_CONFIG=y