Use -oso_prefix to relativize object file paths in debug info with ld64 on macOS

[lf-]

Issue description

Currently absolute paths leak into linked binaries on macOS inside the OSO debug info section, making a lot of binaries nonreproducible since /private/tmp/nix-build-dskfdsjlksdfl/ absolutely can arbitrarily vary between runs. This can be fixed with a ld64 feature called -oso_prefix, about which you can learn more here: https://milen.me/writings/apple-linker-ld64-deterministic-builds-oso-prefix/

[RaitoBezarius]

cc @reckenrode I suppose

[reckenrode]

I tried building hello manually (see below), and it seems to work. It probably should be added an extra input to the stdenv, which would allow it to be dropped when cross-compiling.

$ export NIX_LDFLAGS+=" -oso_prefix $(realpath "$NIX_BUILD_TOP")/"
$ genericBuild
$ nm -a $out/bin/hello | rg OSO
0000000000000000 - 00 0001   OSO hello-2.12.1/./lib/libhello.a(c-strcasecmp.o)
0000000000000000 - 00 0001   OSO hello-2.12.1/./lib/libhello.a(close-stream.o)
0000000000000000 - 00 0001   OSO hello-2.12.1/./lib/libhello.a(closeout.o)
0000000000000000 - 00 0001   OSO hello-2.12.1/./lib/libhello.a(error.o)
0000000000000000 - 00 0001   OSO hello-2.12.1/./lib/libhello.a(exitfail.o)
0000000000000000 - 00 0001   OSO hello-2.12.1/./lib/libhello.a(fpending.o)
0000000000000000 - 00 0001   OSO hello-2.12.1/./lib/libhello.a(getopt.o)
0000000000000000 - 00 0001   OSO hello-2.12.1/./lib/libhello.a(getopt1.o)
0000000000000000 - 00 0001   OSO hello-2.12.1/./lib/libhello.a(localcharset.o)
0000000000000000 - 00 0001   OSO hello-2.12.1/./lib/libhello.a(progname.o)
0000000000000000 - 00 0001   OSO hello-2.12.1/./lib/libhello.a(propername.o)
0000000000000000 - 00 0001   OSO hello-2.12.1/./lib/libhello.a(quotearg.o)
0000000000000000 - 00 0001   OSO hello-2.12.1/./lib/libhello.a(version-etc-fsf.o)
0000000000000000 - 00 0001   OSO hello-2.12.1/./lib/libhello.a(version-etc.o)
0000000000000000 - 00 0001   OSO hello-2.12.1/./lib/libhello.a(xalloc-die.o)
0000000000000000 - 00 0001   OSO hello-2.12.1/./lib/libhello.a(xmalloc.o)
0000000000000000 - 00 0001   OSO hello-2.12.1/src/hello.o