libc: use runtimes dir as base for cmake

[nickdesaulniers]

Fixes: #325

[nickdesaulniers]

@mikhailramalho can you please rm -rf * the contents of the libc-riscv32-qemu-yocto-fullbuild-dbg buildbot's build dir? The cmakecache is stale again as a result of this change.

[mikhailramalho]

Done, cmake is passing now but running the tests is failing: https://lab.llvm.org/buildbot/#/builders/196/builds/2315/steps/4/logs/stdio

Did the path to the tests change? rsync seems to be confused now

[nickdesaulniers]

Did the path to the tests change?

Maybe. I guess I'm surprised to see rsync involved; I would guess that's specific to the libc-riscv32-qemu-yocto-fullbuild-dbg buildbot? Can you reset rsync on the receiving end then if the paths for the tests did change? Perhaps that's necessary for the buildbot whenever resetting the cmake cache?

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I'm also curious about the contents of /home/libcrv32buildbot/cross.sh. Is that the massive command line invocation for qemu? Looks like you set that script for -DCMAKE_CROSSCOMPILING_EMULATOR.