Makefile.m32: delete obsolete options, improve -On [ci skip]

Makefile.m32: delete obsolete options, improve -On [ci skip]

- `-D_AMD64_` has not been necessary for mingw-w64 builds for a long time now.
- `-fno-strict-aliasing` is mentioned for Intel C compiler in autotools, and
  I used this with VxWorks in another project, but otherwise this isn't
  necessary anymore as a default. If a target still needs it, it can be
  added with `CURL_CFLAG_EXTRAS=-fno-strict-aliasing`
- bump up default optimization level to `-O3` (from `-O2`), and also rearrange
  option order so the default can now be overridden via
  `CURL_CFLAG_EXTRAS`.
- delete `-g` (generate debug info) from `CFLAGS` and `-s` from `LDFLAGS`
  (strip debug info). They were working against each other. Now, if someone
  needs debug info, it can be enabled via `CURL_CFLAG_EXTRAS=-g`

Closes #8904