Generate gnu hash for arm only

Fixes build failure caused by:
https://android-review.googlesource.com/#/c/139660

As previously discussed [1], MIPS does not support GNU-style ELF hashes.

[1] https://android-review.googlesource.com/#/c/49282/2/tests/Android.mk

Change-Id: I9a7966eebfd1ef0a587a20b71faefde38e84ab62
diff --git a/libm/Android.mk b/libm/Android.mk
index 092ebd5..2f30c82 100644
--- a/libm/Android.mk
+++ b/libm/Android.mk
@@ -439,7 +439,7 @@
 include $(CLEAR_VARS)
 
 # TODO: This is to work around b/19059885. Remove after root cause is fixed
-LOCAL_LDFLAGS := -Wl,--hash-style=both
+LOCAL_LDFLAGS_arm := -Wl,--hash-style=both
 
 LOCAL_MODULE := libm
 LOCAL_CLANG := $(libm_clang)