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