package/Makefile.in: ban textrels on musl toolchains

musl-libc doesn't support TEXTRELs[1] and programs with TEXTRELs will
crash on start-up under musl.

This patch forbids the use of TEXTRELs on musl toolchains with dynamic
linking.

To verify this patch:

- Delete package/micropython/0001-py-nlrthumb-Make-non-Thumb2-long-jump-workaround-opt.patch
- Build micropython (before v1.23) with a musl toolchain and
  BR2_SHARED_LIBS. The build should abort while linking micropython.

[1]: https://www.openwall.com/lists/musl/2020/09/25/4

Signed-off-by: J. Neuschäfer <j.neuschaefer@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/Makefile.in

@@ -149,6 +149,15 @@ endif
 
 TARGET_LDFLAGS = $(call qstrip,$(BR2_TARGET_LDFLAGS))
 
+# musl's dynamic loader doesn't support DT_TEXTREL, which results in a runtime
+# crash if it gets used. The "-z text" linker option issues a build-time error
+# when DT_TEXREL is used, so we capture the problem earlier.
+#
+# See also: https://www.openwall.com/lists/musl/2020/09/25/4
+ifeq ($(BR2_TOOLCHAIN_USES_MUSL):$(BR2_STATIC_LIBS),y:)
+TARGET_LDFLAGS += -Wl,-z,text
+endif
+
 # By design, _FORTIFY_SOURCE requires gcc optimization to be enabled.
 # Therefore, we need to pass _FORTIFY_SOURCE and the optimization level
 # through the same mechanism, i.e currently through CFLAGS. Passing