build: strip: unify win32 and use the configured $STRIP
- There's no need to force STRIP_BINARIES on windows since --enable-strip (at
configure) already does exactly that, if one wants to.
- Use the contigured $STRIP instead of the native 'strip', useful when
cross building tcc.
- 'make install-strip' now also strips libtcc.dll on windows (it already does
so now with --enable-strip, and previously it always stripped it).
Summary of current strip options for all platforms:
- configure --enable strip -> 'install -s' for the binaries.
- make install-strip: installs and then configured $STRIP the binaries.
- Otherwise -> no stripping.