arm: Use -O2 instead of -O as optimization flag

When using Clang -O is equivalent to -O2, change it -O2 to make it
consistent with other platforms.

Reference: https://clang.llvm.org/docs/ClangCommandLineReference.html#optimization-level

Submitted by:	Daniel Engberg (daniel.engberg.lists@pyret.net)
Reviewed by:	emaste
Differential Revision:	https://reviews.freebsd.org/D21021

share/mk/sys.mk

@@ -166,7 +166,7 @@ CC		?=	c89
 CFLAGS		?=	-O
 .else
 CC		?=	cc
-.if ${MACHINE_CPUARCH} == "arm" || ${MACHINE_CPUARCH} == "mips"
+.if ${MACHINE_CPUARCH} == "mips"
 CFLAGS		?=	-O -pipe
 .else
 CFLAGS		?=	-O2 -pipe