Don't include GNU object attributes when building with clang.

LLVM's MIPS assembler parser does not understand the GNU as
'.gnu_attribute' keyword.  This could be re-enabled if LLVM is updated
in the future.  The desired floating point ABI is already described in
the .MIPS.abiflags section.

Reviewed by:	emaste
Sponsored by:	DARPA / AFRL

lib/csu/mips/crti.S

@@ -1,7 +1,9 @@
 #include <machine/asm.h>
 __FBSDID("$FreeBSD$");
 
+#ifndef __clang__
 	.gnu_attribute 4, 0
+#endif
 	.section .init,"ax",%progbits
 	.align 4
 	.globl	_init

lib/csu/mips/crtn.S

@@ -1,7 +1,9 @@
 #include <machine/asm.h>
 __FBSDID("$FreeBSD$");
 
+#ifndef __clang__
 	.gnu_attribute 4, 0
+#endif
 	.section .init,"ax",%progbits
 	.align 4
 	.set noreorder