Index: uspace/lib/c/arch/amd64/Makefile.common
===================================================================
--- uspace/lib/c/arch/amd64/Makefile.common	(revision d75dc0561c7ce077e18de03f15c1afa664e05d5e)
+++ uspace/lib/c/arch/amd64/Makefile.common	(revision bfdb7c6344e26e53bd81545c5163f53e96517dd6)
@@ -27,5 +27,10 @@
 #
 
-COMMON_CFLAGS += -mno-tls-direct-seg-refs -fno-omit-frame-pointer
+COMMON_CFLAGS += -fno-omit-frame-pointer
+
+# XXX: clang doesn't support this flag, but the optimization is OS-specific,
+#      so it isn't used for amd64-unknown-elf target.
+GCC_CFLAGS += -mno-tls-direct-seg-refs
+
 LFLAGS += --gc-sections