Fix install-strip for cross-compilation

The variable INSTALL_PROGRAM_ENV sets up STRIPPROG for the cross-compiler.

If this is not done, the host 'strip' is used, and fails:

/bin/sh /c/src/repos/binutils-gdb.git/install-sh -c -s gdb.exe \
  /gdb/gdb64-git/bin/$transformed_name.exe
strip.exe:C:/gdb/gdb64-git/bin/_inst.33599_: file format not recognized

With this change, it's fine:

STRIPPROG='x86_64-w64-mingw32-strip' \
  /bin/sh /c/src/repos/binutils-gdb.git/install-sh -c -s gdb.exe \
  /gdb/gdb64-git/bin/$transformed_name.exe

gdb/ChangeLog:

2020-01-01  Hannes Domani  <ssbssa@yahoo.de>

	* Makefile.in: Use INSTALL_PROGRAM_ENV.

gdb/gdbserver/ChangeLog:

2020-01-01  Hannes Domani  <ssbssa@yahoo.de>

	* Makefile.in: Use INSTALL_PROGRAM_ENV.