arm: enable stack-smashing protection

With current generation clang/llvm it can pass all of our tests in
libc/ssp.

While here, remove the extra MACHINE_CPUARCH check for mips. SSP is
included in BROKEN_OPTIONS for this architecture in src.opts.mk, which
is enough to ensure normal builds won't set SSP_CFLAGS.

Reviewed by:	kevans, imp, emaste
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D31400

(cherry picked from commit 1b8db4b4e3)

share/mk/bsd.sys.mk

@@ -239,12 +239,11 @@ CFLAGS.clang+=	 -Qunused-arguments
 # but not yet.
 CXXFLAGS.clang+=	 -Wno-c++11-extensions
 
-.if ${MK_SSP} != "no" && \
-    ${MACHINE_CPUARCH} != "arm" && ${MACHINE_CPUARCH} != "mips"
+.if ${MK_SSP} != "no"
 # Don't use -Wstack-protector as it breaks world with -Werror.
 SSP_CFLAGS?=	-fstack-protector-strong
 CFLAGS+=	${SSP_CFLAGS}
-.endif # SSP && !ARM && !MIPS
+.endif # SSP
 
 # Additional flags passed in CFLAGS and CXXFLAGS when MK_DEBUG_FILES is
 # enabled.

sys/conf/kern.mk

@@ -213,8 +213,7 @@ CFLAGS+=	-fwrapv
 #
 # GCC SSP support
 #
-.if ${MK_SSP} != "no" && \
-    ${MACHINE_CPUARCH} != "arm" && ${MACHINE_CPUARCH} != "mips"
+.if ${MK_SSP} != "no"
 CFLAGS+=	-fstack-protector
 .endif