This provides a structured collection of the source files used in the
compiler-rt builtins library in Bazel.
Normal build rules often don't work for runtime libraries as they may
need to be built for a specific target platform and in an environment
with the associated SDK available to build for that target. Instead,
this PR exports the sources in a structured way that can be used by
downstream users to collect and build these runtimes in a
target-appropriate manner.
Currently, this includes AArch64, AArch32 (with and without VFP),
x86-64, i386, PPC, and RISC-V. Where I could see a useful division of
functionality, those are also exposed.
The rules use over-wide globs to minimize the need to manually update
lists of files or to risk things slipping out of date.
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