build: rearrange -O3 and -f<optimization-option> options
build: rearrange -O3 and -f<optimization-option> options

It turns out that -O3 enabled -fslp-vectorize even if it is
disabled before -O3 on the command line. Rearrange the code
so that -O3 is before the more specific optimization options.

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla