riscv: Do not use cfi_label when building with clang

The .cfi_label is a gas extension not supported by clang.  From a
ziglang discussion [1], it seems that it is not really required.

[1] https://github.com/ziglang/zig/issues/3340

sysdeps/generic/sysdep.h

@@ -55,6 +55,12 @@
 # define cfi_window_save		.cfi_window_save
 # define cfi_personality(enc, exp)	.cfi_personality enc, exp
 # define cfi_lsda(enc, exp)		.cfi_lsda enc, exp
+/* .cfi_label is a gas extension not supported by clang.  */
+# ifndef __clang__
+#  define cfi_label(label)		.cfi_label label
+# else
+#  define cfi_label(label)
+# endif
 
 #else /* ! ASSEMBLER */
 

sysdeps/riscv/start.S

@@ -45,7 +45,7 @@
 ENTRY (ENTRY_POINT)
 	/* Terminate call stack by noting ra is undefined.  Use a dummy
 	   .cfi_label to force starting the FDE.  */
-	.cfi_label .Ldummy
+	cfi_label (.Ldummy)
 	cfi_undefined (ra)
 	call  load_gp
 	mv    a5, a0  /* rtld_fini.  */

sysdeps/unix/sysv/linux/riscv/clone.S

@@ -74,7 +74,7 @@ ENTRY (__thread_start)
 L (thread_start):
 	/* Terminate call stack by noting ra is undefined.  Use a dummy
 	   .cfi_label to force starting the FDE.  */
-	.cfi_label .Ldummy
+	cfi_label (.Ldummy)
 	cfi_undefined (ra)
 
 	/* Restore the arg for user's function.  */

sysdeps/unix/sysv/linux/riscv/clone3.S

@@ -62,7 +62,7 @@ ENTRY(__thread_start_clone3)
 L(thread_start):
 	/* Terminate call stack by noting ra is undefined.  Use a dummy
 	   .cfi_label to force starting the FDE.  */
-	.cfi_label .Ldummy
+	cfi_label (.Ldummy)
 	cfi_undefined (ra)
 
 	/* Restore the arg for user's function and call the user's