Add overlay libc++ for arm64 when x-build

[mathetake]

For envoyproxy/envoy#43826

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[mathetake]

still verifying

[mathetake]

cool working

[phlax]

reviewing further and same comment as before - this is not the right place to put this - it needs to go in toolchains_llvm - sysroot does not carry the c++ libs, just glibc (and some related libs)

not only do i think that this doesnt belong here - it would swell the sysroot for all consumers for an edge case we never had till now

iiuc the underlying problem is of toolchains_llvm only providing libc++ for the arch that it provides a compiler for - so in your case it doesnt provide the arm libc++ because you are using the x86 compiler

really i think this needs to be resolved in toolchains_llvm - its not such an uncommon pattern and they have the machinery to do this cleanly - however, im also aware that that fix is non trivial

the compromise im thinking of is that we publish these libs for x-compile to a separate binary package - with a related repo setup in toolshed - similar to what we do for msan/tsan currently (sans the need to compile anything)

ill get a bot on the case with that now

[mathetake]

the compromise im thinking of is that we publish these libs for x-compile to a separate binary package - with a related repo setup in toolshed - similar to what we do for msan/tsan currently (sans the need to compile anything)

sounds good