linuxulator: rename linux_locore.s to .asm

It is assembled using "${CC} -x assembler-with-cpp", which by convention
(bsd.suffixes.mk) uses the .asm extension.

This is a portion of the review referenced below (D18344).  That review
also renamed linux_support.s to .S, but that is a functional change
(using the compiler's integrated assembler instead of as) and will be
revisited separately.

MFC after:	1 week
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D18344

sys/conf/files.amd64

@@ -45,7 +45,7 @@ linux32_assym.h			optional	compat_linux32		\
 	clean		"linux32_assym.h"
 #
 linux32_locore.o	optional	compat_linux32		\
-	dependency 	"linux32_assym.h $S/amd64/linux32/linux32_locore.s"	\
+	dependency 	"linux32_assym.h $S/amd64/linux32/linux32_locore.asm"	\
 	compile-with	"${CC} -x assembler-with-cpp -DLOCORE -m32 -shared -s -pipe -I. -I$S -Werror -Wall -fPIC -fno-common -nostdinc -nostdlib -Wl,-T$S/amd64/linux32/linux32_vdso.lds.s -Wl,-soname=linux32_vdso.so,--eh-frame-hdr,-warn-common ${.IMPSRC} -o ${.TARGET}" \
 	no-obj no-implicit-rule						\
 	clean		"linux32_locore.o"

sys/conf/files.i386

@@ -32,7 +32,7 @@ linux_assym.h			optional	compat_linux		\
 	clean		"linux_assym.h"
 #
 linux_locore.o			optional	compat_linux		\
-	dependency 	"linux_assym.h $S/i386/linux/linux_locore.s"	\
+	dependency 	"linux_assym.h $S/i386/linux/linux_locore.asm"	\
 	compile-with	"${CC} -x assembler-with-cpp -DLOCORE -shared -s -pipe -I. -I$S -Werror -Wall -fPIC -fno-common -nostdinc -nostdlib -Wl,-T$S/i386/linux/linux_vdso.lds.s -Wl,-soname=linux_vdso.so,--eh-frame-hdr,-warn-common ${.IMPSRC} -o ${.TARGET}" \
 	no-obj no-implicit-rule						\
 	clean		"linux_locore.o"

sys/modules/linux64/Makefile

@@ -29,7 +29,7 @@ OBJS=	${VDSO}.so
 linux_assym.h: linux_genassym.o
 	sh ${SYSDIR}/kern/genassym.sh linux_genassym.o > ${.TARGET}
 
-linux_locore.o: linux_locore.s linux_assym.h
+linux_locore.o: linux_locore.asm linux_assym.h
 	${CC} -x assembler-with-cpp -DLOCORE -shared -mcmodel=small	\
 	-pipe -I. -I${SYSDIR} -Werror -Wall -fno-common -fPIC -nostdinc	\
 	-Wl,-T${SRCTOP}/sys/${MACHINE}/linux/${VDSO}.lds.s	\