I'm testing on two environments:
E1) with dlltool 2.38-1
E2) with dlltool 2.37-3

I see the error in E1 only <g>

on all env, I run 
> strace winebuild... <c/l from makefile> -v -v >& log
> grep exec log

results:
====================
E2) 
pid 12272] execve("/usr/bin/x86_64-w64-mingw32-dlltool", ["/usr/bin/x86_64-w64-mingw32-dllt"..., "-k", "-y", "dlls/iphlpapi/libiphlpapi.delay."..., "-d", "libiphlpapi.delay-625082dd.def", "-m", "i386:x86-64", "--as-flags=--64"], 0x7ffc19487b48 /* 56 vars */) = 0
[pid 12273] execve("/usr/bin/x86_64-w64-mingw32-as", ["/usr/bin/x86_64-w64-mingw32-as", "--64", "-o", "daesh.o", "daesh.s"], 0x7fffa1e12cf8 /* 56 vars */ <unfinished ...>
[pid 12273] <... execve resumed>)       = 0
[pid 12274] execve("/usr/bin/x86_64-w64-mingw32-as", ["/usr/bin/x86_64-w64-mingw32-as", "--64", "-o", "daest.o", "daest.s"], 0x7fffa1e12cf8 /* 56 vars */ <unfinished ...>
[pid 12274] <... execve resumed>)       = 0

====================
E1-without the patch below)
execve("tools/winebuild/winebuild", ["tools/winebuild/winebuild", "-b", "x86_64-w64-mingw32", "-w", "--implib", "-o", "dlls/iphlpapi/libiphlpapi.delay."..., "--export", "/home/eric/wine/wine/dlls/iphlpa"..., "-v", "-v"], 0x7ffc4d598b28 /* 54 vars */) = 0
[pid  9078] execve("/usr/bin/x86_64-w64-mingw32-dlltool", ["/usr/bin/x86_64-w64-mingw32-dllt"..., "-k", "-y", "dlls/iphlpapi/libiphlpapi.delay."..., "-d", "libiphlpapi.delay-6250734a.def", "-m", "i386:x86-64", "--as-flags=--64"], 0x7ffd66663948 /* 54 vars */) = 0
[pid  9079] execve("/usr/bin/x86_64-w64-mingw32-as", ["/usr/bin/x86_64-w64-mingw32-as", "--64", "-o", "iphlpapi_dll_h.o", "iphlpapi_dll_h.s"], 0x7ffd4c406e08 /* 54 vars */ <unfinished ...>
[pid  9079] <... execve resumed>)       = 0
[pid  9080] execve("/usr/bin/x86_64-w64-mingw32-as", ["/usr/bin/x86_64-w64-mingw32-as", "--64", "-o", "iphlpapi_dll_t.o", "iphlpapi_dll_t.s"], 0x7ffd4c406e08 /* 54 vars */ <unfinished ...>
[pid  9080] <... execve resumed>)       = 0

====================
E1-with the patch above)
execve("tools/winebuild/winebuild", ["tools/winebuild/winebuild", "-b", "x86_64-w64-mingw32", "-w", "--implib", "-o", "dlls/iphlpapi/libiphlpapi.delay."..., "--export", "/home/eric/wine/wine/dlls/iphlpa"..., "-v", "-v"], 0x7ffd5925c228 /* 54 vars */) = 0
[pid  9601] execve("/usr/bin/x86_64-w64-mingw32-dlltool", ["/usr/bin/x86_64-w64-mingw32-dllt"..., "-k", "-y", "dlls/iphlpapi/libiphlpapi.delay."..., "-d", "libiphlpapi.delay-6250774f.def", "-t", "9600\n", "-m", "i386:x86-64", "--as-flags=--64"], 0x7fff84ee6468 /* 54 vars */) = 0
[pid  9602] execve("/usr/bin/x86_64-w64-mingw32-as", ["/usr/bin/x86_64-w64-mingw32-as", "--64", "-o", "9600\nh.o", "9600\nh.s"], 0x7ffdc7cf0c98 /* 54 vars */ <unfinished ...>
[pid  9602] <... execve resumed>)       = 0
[pid  9603] execve("/usr/bin/x86_64-w64-mingw32-as", ["/usr/bin/x86_64-w64-mingw32-as", "--64", "-o", "9600\nt.o", "9600\nt.s"], 0x7ffdc7cf0c98 /* 54 vars */ <unfinished ...>
[pid  9603] <... execve resumed>)       = 0

to summarize, binutils 2.38 no longer generates a temp file name dependant on the pid... using the -t options forces back the regular behavior...

Bernhard, Sveinar: could you also look at the dlltool --version output to see if this converges to 2.38*

looks I have to dig further into binutils source code