build: use ccache in make-v8.sh on ppc64le and s390x

[richardlau]

If ccache is available, use it during V8 builds on ppc64le and s390x.
Only create the gcc and g++ shims if necessary.

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Stumbled across https://chromium.googlesource.com/chromium/src/+/HEAD/docs/ccache_mac.md while setting up new RHEL 8 instances on the CI.

I attempted to also enable ccache for the else branch of make-v8.sh, e.g. for Linux x64, but while this did work on the new RHEL 8 x64 CI instance I've been setting up, this fails on test-nearform_intel-ubuntu1604-x64-1 (the machine currently used for label benchmark-ubuntu1604-intel-64 in the job) with lots of errors of this type:

clang++: error: argument unused during compilation:

I figure we can enable this for ppc64le and s390x first and potentially revisit for x64 later on. We've procrastinated over upgrading the benchmark machines (i.e. test-nearform_intel-ubuntu1604-x64-1) from Ubuntu 16.04 for a while (nodejs/build#2656) and this could be another reason to do so.