Refactor llvm_config.mk and support the 2nd arch

1. Following the setup of gcc in build/core/combo/,
we added the [HOST|TARGET]_<arch>.mk clang config files,
and load only the configs needed by the current product.
2. Added support for the 2nd arch.

Change-Id: I2a383418a9688a050b39492f8e489d40eeeb5f2d
diff --git a/core/clang/arm.mk b/core/clang/arm.mk
new file mode 100644
index 0000000..28d87bb
--- /dev/null
+++ b/core/clang/arm.mk
@@ -0,0 +1,27 @@
+# Clang flags for arm arch, target or host.
+
+CLANG_CONFIG_arm_EXTRA_ASFLAGS :=
+
+CLANG_CONFIG_arm_EXTRA_CFLAGS := \
+  -mllvm -arm-enable-ehabi
+
+CLANG_CONFIG_arm_EXTRA_LDFLAGS :=
+
+# Include common unknown flags
+CLANG_CONFIG_arm_UNKNOWN_CFLAGS := \
+  $(CLANG_CONFIG_UNKNOWN_CFLAGS) \
+  -mthumb-interwork \
+  -fgcse-after-reload \
+  -frerun-cse-after-loop \
+  -frename-registers \
+  -fno-builtin-sin \
+  -fno-strict-volatile-bitfields \
+  -fno-align-jumps \
+  -Wa,--noexecstack
+
+define subst-clang-incompatible-arm-flags
+  $(subst -march=armv5te,-march=armv5t,\
+  $(subst -march=armv5e,-march=armv5,\
+  $(subst -mcpu=cortex-a15,-march=armv7-a,\
+  $(1))))
+endef