arc: disable obstack, not usable with newer binutils/gdb

target/arc/uclibc-ng.config

@@ -88,7 +88,7 @@ MALLOC=y
 # MALLOC_SIMPLE is not set
 # MALLOC_STANDARD is not set
 MALLOC_GLIBC_COMPAT=y
-UCLIBC_HAS_OBSTACK=y
+# UCLIBC_HAS_OBSTACK is not set
 UCLIBC_DYNAMIC_ATEXIT=y
 COMPAT_ATEXIT=y
 UCLIBC_HAS_UTMPX=y