fix(termtris): run the 2ine loader under qemu-i386 when the kernel lacks 16-bit LDT

The r5.01 tools are 16-bit OS/2 NE programs.  A Linux kernel built
without CONFIG_X86_16BIT (this box's cloud kernel) rejects modify_ldt
for 16-bit segments, so lx_loader aborts at its first segment even
though it is a 32-bit binary.  Probe for that failure and use
qemu-i386, whose linux-user modify_ldt supports 16-bit segments.
LX_RUNNER overrides the choice.  The toolchain output is unchanged:
the build completes in ~10 s and the COFF differs only in its header
timestamp.

apps/termtris/build.sh

@@ -32,6 +32,26 @@ if [ ! -x "$WORK/gspac.exe" ]; then
 	unzip -oq "$Z1" -d "$WORK"
 	unzip -oq "$Z2" -d "$WORK"
 fi
+
+# The r5.01 tools are 16-bit OS/2 NE programs, which 2ine runs through 16-bit
+# LDT segments.  A Linux kernel built without CONFIG_X86_16BIT rejects those
+# (modify_ldt returns EINVAL), so the loader aborts at its first segment even
+# though it is a 32-bit binary.  Detect that with a probe run and, when needed,
+# drive the same loader through qemu-i386, whose linux-user emulation does
+# implement the 16-bit LDT.  Set LX_RUNNER to a command prefix (for example
+# "qemu-i386") to force one; leave it unset to auto-detect.
+if [ -z "${LX_RUNNER:-}" ]; then
+	if ( cd "$WORK" && "$LX" ./gspac.exe ) 2>&1 |
+		grep -q 'failed to allocate LDT selector'; then
+		if command -v qemu-i386 >/dev/null 2>&1; then
+			LX_RUNNER=qemu-i386
+		else
+			echo "build.sh: this kernel has no 16-bit LDT support and qemu-i386 is not installed" >&2
+			echo "build.sh: install qemu-user, or set LX_RUNNER to a working runner" >&2
+			exit 1
+		fi
+	fi
+fi
 for so in "$NINE"/lib*.so; do
 	[ -e "$so" ] && ln -sf "$so" "$WORK/"
 done
@@ -78,7 +98,7 @@ run() {
 	shift
 	i=0
 	while [ "$i" -lt 5 ]; do
-		( cd "$WORK" && "$LX" "$@" ) >/dev/null 2>&1 || true
+		( cd "$WORK" && ${LX_RUNNER} "$LX" "$@" ) >/dev/null 2>&1 || true
 		if [ -f "$WORK/$_out" ]; then
 			return 0
 		fi