gnustep-base, gnustep-gui, gnustep-back: Build on clang64

[qmfrederik]

gnustep-base, gnustep-gui and gnustep-back were disabled on clang64 because of linker issues. Linking the libraries failed with this error:

ld.lld: error: libc++.dll.a(libc++.dll): .idata$4 should not refer to special section 0

This appears to be because the GNUstep build process defines -Wl,--whole-archive in target.make. Prefixing the reference to libc++ with -Wl,--no-whole-archive manages to work around this.

I'm not sure if this is a bug in libc++.dll.a; .idata$4 appears to be empty:

# objdump -s -j '.idata$4' /clang64/lib/libc++.dll.a  | grep idata -A 2
Contents of section .idata$4:
 0000 00000000 00000000                    ........

#21017 (comment) has some more details.