makefile.m32: add support for custom ARCH [ci skip]

makefile.m32: add support for custom ARCH [ci skip]

When building curl for target platform other than x64 and x86, it is now
possible to pass `ARCH=custom`, that will omit all hardcoded logic for
setting up CFLAGS/LDFLAGS/RCFLAGS for these platforms, and let these be
customized via `CURL_CFLAG_EXTRAS`, `CURL_LDFLAG_EXTRAS`, and a newly
added one for the resource compiler: `CURL_RCFLAG_EXTRAS`.

This makes it possible to use `makefile.m32` to build for ARM64 for
example.

Reviewed-by: Daniel Stenberg
Closes #9092