stop building arm64 kernels with -mcpu=cortex-a57

[Jonathan Gray]

Ask for a generic armv8-a encoding rather than one based on and tuned
for cortex-a57.

Index: Makefile.arm64
===================================================================
RCS file: /cvs/src/sys/arch/arm64/conf/Makefile.arm64,v
retrieving revision 1.4
diff -u -p -r1.4 Makefile.arm64
--- Makefile.arm64      25 Jan 2017 11:15:07 -0000      1.4
+++ Makefile.arm64      5 Feb 2017 10:10:31 -0000
@@ -28,7 +28,7 @@ CWARNFLAGS=   -Werror -Wall -Wimplicit-fun
                -Wno-constant-conversion -Wno-address-of-packed-member \
                -Wframe-larger-than=2047
 
-CMACHFLAGS=    -mcpu=cortex-a57+nofp+nosimd \
+CMACHFLAGS=    -march=armv8-a+nofp+nosimd \
                -fno-omit-frame-pointer -mno-omit-leaf-frame-pointer \
                -ffixed-x18
 CMACHFLAGS+=   -ffreestanding ${NOPIE_FLAGS}