musl: build broken on Thumb, use ARM mode

Building the musl C library on Thumb (Thumb1, not Thumb2), fails with:

 {standard input}:20: Error: only lo regs allowed with immediate -- `mov fp,#0'
 {standard input}:21: Error: only lo regs allowed with immediate -- `mov lr,#0'
 {standard input}:25: Error: unshifted register required -- `and ip,a1,#-16'

Since there are no cores that we support that are Thumb1 only, use the
same solution as the one used by glibc: build the C library in ARM
mode.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/musl/musl.mk

@@ -23,6 +23,12 @@ MUSL_ADD_TOOLCHAIN_DEPENDENCY = NO
 
 MUSL_INSTALL_STAGING = YES
 
+# Thumb build is broken, build in ARM mode, since all architectures
+# that support Thumb1 also support ARM.
+ifeq ($(BR2_ARM_INSTRUCTIONS_THUMB),y)
+MUSL_EXTRA_CFLAGS += -marm
+endif
+
 define MUSL_CONFIGURE_CMDS
 	(cd $(@D); \
 		$(TARGET_CONFIGURE_OPTS) \