musl: Build only in ARM mode when using clang

clang 5.0 has a codegen bug when compiling musl in thumb2 mode see

https://bugs.llvm.org/show_bug.cgi?id=34165

(From OE-Core rev: 3b4182968c5a1271748002f0e0264f54643a9f1c)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/recipes-core/musl/musl_git.bb

@@ -28,6 +28,8 @@ export CROSS_COMPILE="${TARGET_PREFIX}"
 
 LDFLAGS += "-Wl,-soname,libc.so"
 
+ARM_INSTRUCTION_SET_toolchain-clang = "arm"
+
 CONFIGUREOPTS = " \
     --prefix=${prefix} \
     --exec-prefix=${exec_prefix} \