improve i386 inline syscall asm on non-broken compilers

we have to avoid using ebx unconditionally in asm constraints for
i386, because gcc 3 and 4 and possibly other simplistic compilers
(pcc?) implement PIC via making ebx a fixed-use register, and disallow
its use for anything else. rather than hard-coding knowledge of which
compilers work (at least gcc 5+ and clang), perform a configure test;
this should give us the good codegen on any new compilers we don't yet
know about.

swapping ebx and edx is kept for 1- and 2-arg syscalls because it
avoids having any spills/stack-frame at all in small functions. for
6-arg, if ebx is directly usable, the complex shuffling introduced in
commit c8798ef974 can be avoided, and
ebp can be loaded the same way ebx is in 5-arg syscalls for compilers
that don't support direct use of ebx.

configure

@@ -573,6 +573,20 @@ printf "using compiler runtime libraries: %s\n" "$LIBCC"
 SUBARCH=
 t="$CFLAGS_C99FSE $CPPFLAGS $CFLAGS"
 
+if test "$ARCH" = "i386" ; then
+printf "checking whether compiler can use ebx in PIC asm constraints... "
+cat > "$tmpc" <<EOF
+int foo(int x) { __asm__ ( "" : "+b"(x) ); return x; }
+EOF
+if $CC $CFLAGS_C99FSE $CPPFLAGS $CFLAGS -fPIC \
+  -c -o /dev/null "$tmpc" >/dev/null 2>&1 ; then
+printf "yes\n"
+else
+printf "no\n"
+CFLAGS_AUTO="$CFLAGS_AUTO -DBROKEN_EBX_ASM"
+fi
+fi
+
 if test "$ARCH" = "x86_64" ; then
 trycppif __ILP32__ "$t" && ARCH=x32
 fi