arm: enable ssp

[mhorne]

Summary

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. The SSP
option is marked BROKEN on this architecture in bsd.opts.mk, which is
enough to ensure normal builds won't set SSP_CFLAGS.

[emaste]

Nice cleanup, and BROKEN_OPTIONS or default off is better than distributed special cases.