Optimise for small code by default

My tests with running shbench[*] on ksh binaries compiled by clang
and gcc yielded no performance difference between compiling with
'-O2' and '-Os'. So we might as well reduce ksh's size and memory
footprint by default.

[*] http://fossil.0branch.com/csb/
    https://github.com/ksh-community/shbench

src/cmd/INIT/make.probe:
- Change default gcc optimisation level from -O2 to -Os.
- Change default non-gcc optimisation level from -O to -Os.

src/cmd/INIT/make.probe

@@ -18,7 +18,7 @@ probe_ccs="strip size nm ld ar" # longest to shortest
 probe_debug="-g"
 probe_dll="'-G 0' -Wc,dll,exportall,longname,rent -Wc,exportall -dynamic $cc_dll_def"
 probe_export_dynamic="-rdynamic -export-dynamic -Wl,-export-dynamic -Wl,-E -bexpall -force_flat_namespace"
-probe_gcc_optimize="-O2"
+probe_gcc_optimize="-Os"
 probe_gcc_version="*[Gg][Cc][Cc]*"
 probe_include_local="'-ignore-source-dir -iquote' -iquote -I-"
 probe_ldlazy='-zlazyload -znolazyload -Wl,-zlazyload -Wl,-znolazyload'
@@ -34,7 +34,7 @@ probe_lib_all_undef="-all -notall -all -none -Bwhole-archive -Bno-whole-archive
 probe_lib_multiple="-Wl,-zmuldefs"
 probe_libdir="shlib lib"
 probe_nmflags="'' -p -B"
-probe_optimize="-O"
+probe_optimize="-Os"
 probe_pic="-Kpic -KPIC -fpic -fPIC -pic -PIC +z +Z"
 probe_no_protect="'-fno-stack-protector -fno-stack-protector-all' -GS-"
 probe_readonly="-R -Krodata -xMerge -Wa,-r"