ARC: Strip libgcc debug information when generating BTF

ARC links against the compiler's libgcc archive. The ARC GNU 2023.03
toolchain ships this library with debug information, including complex
floating-point types in integer arithmetic helpers such as __udivdi3.
These types reach vmlinux even though the kernel does not use complex
floating-point arithmetic.

When floating-point BTF encoding is enabled, pahole rejects those types:

  Complex, interval and imaginary float types are not supported
  Error while encoding BTF.
  FAILED: load BTF from .tmp_vmlinux1.BTF.1: No such file or directory

KernelCI's upgrade to pahole 1.31 exposed this long-standing problem as
failed ARC builds. As of 1.32, pahole still rejects these types, and
older versions will stay in use, so the kernel build has to keep them
out of vmlinux. Commit f122668ddcce ("ARC: Add eBPF JIT support")
documented the same failure along with a workaround of disabling
floating-point BTF encoding, but that also discards supported
floating-point types from kernel BTF.

For CONFIG_DEBUG_INFO_BTF, generate a debug-stripped copy of the selected
libgcc archive in the output tree and link that copy. Preserve the archive
structure so that only needed helpers are linked, and use the normal
Kbuild objcopy rule to track input and command changes. The trade-off is
that vmlinux in BTF-enabled builds no longer carries DWARF for the libgcc
helpers. Debug information for kernel code, the installed toolchain and
non-BTF builds stay unchanged.

Reproduced and tested with ARC GCC 12.2.1 and pahole 1.31 and 1.32 using
an ARCv2 tinyconfig with BPF_SYSCALL and DWARF4/BTF enabled. Before and
after vmlinux.o files are identical after stripping debug information.

Fixes: db16c1fe92d7 ("bpf: Generate BTF_KIND_FLOAT when linking vmlinux")
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Alan Maguire (Oracle) <almagui@kernel.org>
Link: https://github.com/kernelci/kernelci-core/commit/e71b8c9f42253180596e734c18965a5aad80c8c5
Link: https://patch.msgid.link/20261015154044.4058618-1-sashal@kernel.org

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