Set higher timeouts for Misc-C++ on AArch64

When running the test-suite on AArch64 at -O0, we sometimes get timeouts
in stepanov-v1p2. It usually takes more than 450s to run it, and the
threshold is only 500s. Increase it to 750s.

We're modifying the Makefiles because this is what the AArch64 buildbots
currently use.

Similar to r305671, which increased the timeouts for PAQ8p and SciMark-2.

llvm-svn: 308165
1 file changed