BUILD: makefile: get rid of the CPU variable

BUILD: makefile: get rid of the CPU variable

The CPU variable, when used, is almost always exclusively used with
"generic" to disable any CPU-specific optimizations, or "native" to
enable "-march=native". Other options are not used and are just making
CPU_CFLAGS more confusing.

This commit just drops all pre-configured variants and replaces them
with documentation about examples of supported options. CPU_CFLAGS is
preserved as it appears that it's mostly used as a proxy to inject the
distro's CFLAGS, and it's just empty by default.

The CPU variable is checked, and if set to anything but "generic", it
emits a warning about its deprecation and invites the user to read
INSTALL.

Users who would just set CPU_CFLAGS will be able to continue to do so,
those who were using CPU=native will have to pass CPU_CFLAGS=-march=native
and those who were passing one of the other options will find it in the
doc as well.

Note that this also removes the "CPU=" line from haproxy -vv, that most
users got used to seeing set to "generic" or occasionally "native"
anyway, thus that didn't provide any useful information.