Gcc uses the same -march= for both -m32 and -m64 on x86-64 unless
--with-arch-32= is used. There is no need for -march=i486 to compile
32-bit libatomic on x86-64.
PR target/70454
* configure.tgt (XCFLAGS): Don't add -march=i486 to compile
32-bit x86 target library on x86-64.
From-SVN: r235287
