configure: replace -Os with equivalent based on -O2

aside from the documented differences, which are the contents of this
patch, GCC's -Os also has hard-coded unwanted behaviors which are
impossible to override, like refusing to strength-reduce division by a
constant to multiplication, presumably because the div saves a couple
bytes of code. for this reason, getting rid of -Os and switching to an
equivalent default optimization profile based on -O2 has been a
long-term goal.

as follow-ups, it may make sense to evaluate which of these variations
from -O2 actually do anything useful, and eliminate the ones which are
not helpful or which throw away performance for insignificant size
savings. but for now, I've replicated -Os as closely as possible to
provide a baseline for such evaluation.

configure

@@ -444,7 +444,20 @@ xno|x) printf "disabled\n" ; optimize=no ;;
 *) printf "custom\n" ;;
 esac
 
-test "$optimize" = no || tryflag CFLAGS_AUTO -Os || tryflag CFLAGS_AUTO -O2
+if test "$optimize" = no ; then :
+else
+tryflag CFLAGS_AUTO -O2
+tryflag CFLAGS_AUTO -fno-align-jumps
+tryflag CFLAGS_AUTO -fno-align-functions
+tryflag CFLAGS_AUTO -fno-align-loops
+tryflag CFLAGS_AUTO -fno-align-labels
+tryflag CFLAGS_AUTO -fira-region=one
+tryflag CFLAGS_AUTO -fira-hoist-pressure
+tryflag CFLAGS_AUTO -freorder-blocks-algorithm=simple \
+|| tryflag CFLAGS_AUTO -fno-reorder-blocks
+tryflag CFLAGS_AUTO -fno-prefetch-loop-arrays
+tryflag CFLAGS_AUTO -fno-tree-ch
+fi
 test "$optimize" = yes && optimize="internal,malloc,string"
 
 if fnmatch 'no|size' "$optimize" ; then :