Fix release and standards build flags (re: 35672208, aa4669ad)

bin/package, src/cmd/INIT/package.sh:
- CCFLAGS overwrites the autodetected optimisation flags (e.g. -Os)
  if set. Unfortunately, that also happened when we added something
  to CCFLAGS for a release build or to add an extra flag needed by
  Solaris. The fix is to use a new flags variable (KSH_RELFLAGS)
  instead. This needs to be done in a different place as it needs
  to be added to the mamake command as an assignment argument.
- Remove the Solaris CCFLAGS hack; see features/common below.

src/*/*/Mamfile:
- Add ${KSH_RELFLAGS} to all the compiler commands.

src/lib/libast/features/common:
- Enable POSIX standard on Solaris (i.e.: if __sun is defined) by
  defining _XPG6 directly in the feature test that generates
  ast_std.h, which is indirectly included by everything. This
  removes the need to pass -D_XPG6 via CCFLAGS. (Doing so
  automatically with gcc was not otherwise possible.)

src/cmd/INIT/cc.sol11.*:
- No longer pass -D_XPG6, as per above.

src/cmd/INIT/cc.sol11.i386

@@ -1,4 +1,4 @@
-: solaris.i386 cc wrapper for reasonable ansi C defaults and 32 bit : 2021-01-16 :
+: solaris.i386 cc wrapper for reasonable ansi C defaults and 32 bit : 2021-01-17 :
 
 HOSTTYPE=sol11.i386
 
@@ -13,4 +13,6 @@ case ${CC_EXPLICIT:=$CC} in
 	CC_EXPLICIT=cc
 esac
 
-$CC_EXPLICIT -m32 -xc99 -D_XPG6 "$@"
+# Note: the _XPG6 macro is now defined in src/lib/libast/features/common
+
+$CC_EXPLICIT -m32 -xc99 "$@"