Remove -Wa,noexecstack from unsupported arm clang flags

This was marked as unsupported only on arm[64], even though it works.
For soong, we want to keep a single list of unsupported clang flags
for all architectures.

Change-Id: I9c4ae0f255c549d2346ba7abae57e22c86ddcc61
diff --git a/core/clang/arm.mk b/core/clang/arm.mk
index e66aa6c..4053bb2 100644
--- a/core/clang/arm.mk
+++ b/core/clang/arm.mk
@@ -29,8 +29,7 @@
   -fno-partial-inlining \
   -fno-strict-volatile-bitfields \
   -fno-tree-copy-prop \
-  -fno-tree-loop-optimize \
-  -Wa,--noexecstack
+  -fno-tree-loop-optimize
 
 define subst-clang-incompatible-arm-flags
   $(subst -march=armv5te,-march=armv5t,\