gdb build error under msys+mingw: strip can't handle gdb-add-index.exe

[Eli Zaretskii]

> From: asmwarrior <asmwarrior@gmail.com>
> Date: Tue, 24 Jul 2018 22:13:43 +0800
> 
> if test "x" != x; \
>         then \
>           transformed_name=`t='s,y,y,'; \
>                             echo gcore | sed -e "$t"` ; \
>                   if test "x$transformed_name" = x; then \
>                     transformed_name=gcore ; \
>                   else \
>                     true ; \
>                   fi ; \
>                   /bin/sh ../../binutils-gdb/gdb/../mkinstalldirs /d/code/build_gdb/bin2017/mingw/bin ; \
>                   /bin/install -c gcore \
>                           /d/code/build_gdb/bin2017/mingw/bin/$transformed_name;
>  \
>         fi
> transformed_name=`t='s,y,y,'; \
>                           echo gdb-add-index | sed -e "$t"` ; \
>         if test "x$transformed_name" = x; then \
>           transformed_name=gdb-add-index ; \
>         else \
>           true ; \
>         fi ; \
>         install -s ../../binutils-gdb/gdb/contrib/gdb-add-index.sh \
>                 /d/code/build_gdb/bin2017/mingw/bin/$transformed_name.exe
> D:\code\i686-5.4.0-release-posix-dwarf-rt_v5-rev0\mingw32\bin\strip.exe:d:/code/
> build_gdb/bin2017/mingw/bin/gdb-add-index.exe: File format not recognized
> install: strip failed
> make[1]: *** [install-only] Error 1
> make[1]: Leaving directory `/d/code/build_gdb/mybuildcpp2/gdb'
> make: *** [install] Error 2
> make: Leaving directory `/d/code/build_gdb/mybuildcpp2/gdb'

AFAIK, this script should not be installed by the MinGW build because
it is useless on non-ELF platforms.