[compiler-rt][cmake] Don't reset CAN_TARGET_${arch} on every cmake invocation.

Allowing this variable to be cached makes it possible to repair the MIPS
buildbots in lieu of either fixing the mips64 sanitizer issues or fixing the
detection of mips64 support (which I think was changed by r268977 but didn't
take effect on this buildbot until the last couple days) so that it returns to
not being built on these buildbots.

llvm-svn: 276402