kbuild: remove unnecessary adjustment for Cygwin

"SFX = .exe" was originally added for Cygwin environment.

It is true that GCC on Cygwin spits executables with .exe extention.

For example,

  gcc -o foo foo.c

will generate "foo.exe", not "foo".

But GNU make is also nicely adjusted for Cygwin.

For example,

  foo: foo.c
          gcc -o $@ $<

will compare the timestamp between "foo.exe" and "foo.c".

You do not have to tweak Makefiles like this:

  foo$(SFX): foo.c
          gcc -o $@ $<

And "make clean" works as well without adjustment for Cygwin because
the command "rm foo" on Cygwin will delete both "foo" and "foo.exe".

In conclusion, makefiles do not need special care for Cygwin.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>