curl/system.h: support more architectures

The long list of architectures in include/curl/system.h is annoying to
maintain, and needs to be extended for each and every architecture to
support.

Instead, let's rely on the __SIZEOF_LONG__ define of the gcc compiler
(we are in the GNUC condition anyway), which tells us if long is 4
bytes or 8 bytes.

This fixes the build of libcurl 7.55.0 on architectures such as
OpenRISC or ARC.

Closes #1766

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>