Mark binutils-related conflicts

[sethrj]

When binutils builds on macOS, it omits ld and other key components. Additionally, gcc+binutils %clang sends invalid arguments to the linker since it assumes it's being built with GCC. This MR:

• marks binutils+gold as conflicting with platform=darwin since ld.gold doesn't actually get built in this case;
• marks gcc+binutils on darwin as well, for the same reasons
• marks gcc+binutils+nvptx as conflicting since +binutils requires bootstrapping and +nvptx explicitly disables it.

Configure errors when building with %clang

configure: error: C compiler cannot create executables
See `config.log' for more details.
checking for C compiler default output file name... checking for suffix of object files... checking for C compiler default output file name... make[2]: *** [configure-stage1-libdecnumber] Error 77

where closer inspection of spack-src/spack-build/libdecnumber/config.log shows

configure:2649: /ornldev/code/spack/lib/spack/env/clang/clang -g   -Wl,-rpath,/ornldev/code/spack/opt/spack/clang-11.0.0-apple/gcc/f2gezvp/lib -Wl,-rpath,/ornldev/code/spack/opt/spack/clang-11.0.0-apple/gcc/f2gezvp/lib64 -Wl,-rpath,/ornldev/code/spack/opt/spack/clang-11.0.0-apple/binutils/alksglg/lib -Wl,-rpath,/ornldev/code/spack/opt/spack/clang-11.0.0-apple/gmp/icsy6fw/lib -Wl,-rpath,/ornldev/code/spack/opt/spack/clang-11.0.0-apple/isl/uvsapsa/lib -Wl,-rpath,/ornldev/code/spack/opt/spack/clang-11.0.0-apple/libiconv/jg6ekuh/lib -Wl,-rpath,/ornldev/code/spack/opt/spack/clang-11.0.0-apple/mpc/eam5ugi/lib -Wl,-rpath,/ornldev/code/spack/opt/spack/clang-11.0.0-apple/mpfr/pzmx6wv/lib -Wl,-rpath,/ornldev/code/spack/opt/spack/clang-11.0.0-apple/zlib/xj36ejk/lib -static-libstdc++ -static-libgcc -Wl,-no_pie  conftest.c  >&5
clang: error: unsupported option '-static-libgcc'
configure:2653: $? = 1
configure:2690: result: 
configure: failed program was:
| /* confdefs.h */
| #define PACKAGE_NAME "libdecnumber"
| #define PACKAGE_TARNAME "libdecnumber"
| #define PACKAGE_VERSION " "
| #define PACKAGE_STRING "libdecnumber  "
| #define PACKAGE_BUGREPORT "gcc-bugs@gcc.gnu.org"
| #define PACKAGE_URL ""
| /* end confdefs.h.  */
| 
| int
| main ()
| {
| 
|   ;
|   return 0;
| }
configure:2696: error: in `/private/var/folders/fy/x2xtwh1n7fn0_0q2kk29xkv9vvmbqb/T/s3j/spack-stage/spack-stage-gcc-8.3.0-f2gezvpdyzaxh3drmmiz6i2ou22vgbip/spack-src/spack-build/libdecnumber':
configure:2700: error: C compiler cannot create executables
See `config.log' for more details.

After fixing that error but still using +binutils

configure: error: cannot execute: /ornldev/code/spack/opt/spack/clang-11.0.0-apple/binutils/alksglg/bin/ld: check --with-ld or env. var. DEFAULT_LINKER

[adamjstewart]

I'm fine with these changes (I've never used +binutils or +gold), but I would like to see someone else review this before merging.

[sethrj]

The +binutils stuff was due to @eschnett four years ago, maybe he could comment?

[eschnett]

I used to have problems building gcc on MacOS. These days, the default build options for gcc work for me. I don't build binutils any more (on MacOS), so removing support for it is fine by me.