When building the kernel for amd64 with clang, don't attempt to use the

-frename-registers option, as it is not supported.  No change for builds
with gcc.

MFC after:	1 week

sys/conf/kern.pre.mk

@@ -34,8 +34,12 @@ _MINUS_O=	-O2
 .endif
 .endif
 .if ${MACHINE_CPUARCH} == "amd64"
+.if ${CC:T:Mclang} != "clang"
 COPTFLAGS?=-O2 -frename-registers -pipe
 .else
+COPTFLAGS?=-O2 -pipe
+.endif
+.else
 COPTFLAGS?=${_MINUS_O} -pipe
 .endif
 .if !empty(COPTFLAGS:M-O[23s]) && empty(COPTFLAGS:M-fno-strict-aliasing)