pahole: enable --reproducible_build when SOURCE_DATE_EPOCH is set

[martinetd]

The SOURCE_DATE_EPOCH environment variable is used in reproducible
builds to set build timestamps and is generally synonymous with
reproducible builds: instead of making users figure out how to pass
pahole flags (e.g. explicitly setting PAHOLE_FLAGS in linux) just assume
that the user wants a reproducible build if this variable is set.

The impact on build time appears to be minimal enough even if we
incorrectly make this decision (on a Ryzen 7 7735HS):

$ hyperfine 'sh pahole-test -j --reproducible_build' 'sh pahole-test -j1' 'sh pahole-test -j' -p 'cp .tmp_vmlinux.orig .tmp_vmlinux.btf' -m 4
Benchmark 1: sh pahole-test -j --reproducible_build
  Time (mean ± σ):      3.991 s ±  0.045 s    [User: 7.223 s, System: 3.741 s]
  Range (min … max):    3.950 s …  4.042 s    4 runs

Benchmark 2: sh pahole-test -j1
  Time (mean ± σ):      7.083 s ±  0.095 s    [User: 4.805 s, System: 2.242 s]
  Range (min … max):    6.964 s …  7.191 s    4 runs

Benchmark 3: sh pahole-test -j
  Time (mean ± σ):      3.858 s ±  0.089 s    [User: 13.447 s, System: 7.078 s]
  Range (min … max):    3.763 s …  3.978 s    4 runs

Summary
  sh pahole-test -j ran
    1.03 ± 0.03 times faster than sh pahole-test -j --reproducible_build
    1.84 ± 0.05 times faster than sh pahole-test -j1

Where pahole-test is the pahole command of the linux build (minus -j):

LLVM_OBJCOPY=objcopy pahole -J --btf_gen_floats --lang_exclude=rust --skip_encoding_btf_inconsistent_proto --btf_gen_optimized .tmp_vmlinux.btf "$@"

And .tmp_vmlinux.orig was the .tmp_vmlinux.btf binary before being
processed:

ld -m elf_x86_64 -z noexecstack --no-warn-rwx-segments --emit-relocs --discard-none -z max-page-size=0x200000 --build-id=sha1 --orphan-handling=warn --script=./arch/x86/kernel/vmlinux.lds -o .tmp_vmlinux.btf --whole-archive vmlinux.a .vmlinux.export.o init/version-timestamp.o --no-whole-archive --start-group --end-group

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FWIW this will replace the old patch we had in nixpkgs that used to forcefully set -j1 instead: https://github.com/NixOS/nixpkgs/blob/master/pkgs/by-name/pa/pahole/threading-reproducibility.patch

happy to adjust anything!