In order to run hermetic tests downstream (#145349), we need startup
code. This is based on the crt0 that is present in Arm Toolchain:
5446a3cbf4/arm-software/embedded/llvmlibc-support/crt0.
Note that some tests do fail due to lack of hardware setup, which will
be implemented at a later time.
Most of the CMake/structure is a copy from the existing code in both
`libc/startup/linux` and `libc/startup/gpu`. It is required to build an
object file to be linked with.
Also add header files for init/fini such that crt0 can use the provided
symbols.
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Co-authored-by: Michael Jones <michaelrj@google.com>
