ARC: build: allow users to specify -mcpu

kernel build system used to add -mcpu for each ARC ISA as default.
These days there are versions and varaints of ARC HS cores some of which
have specific -mcpu options to fine tune / optimize generated code.

So allow users/external build systems to specify their own -mcpu

This will be used in future patches for HSDK-4xD board support which
uses specific -mcpu to utilize dual issue scheduling of the core.
Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
[abrodkin/vgupta: rewrote changelog]

arch/arc/Kconfig

@@ -170,6 +170,15 @@ config ARC_CPU_HS
 
 endchoice
 
+config ARC_TUNE_MCPU
+	string "Override default -mcpu compiler flag"
+	default ""
+	help
+	  Override default -mcpu=xxx compiler flag (which is set depending on
+	  the ISA version) with the specified value.
+	  NOTE: If specified flag isn't supported by current compiler the
+	  ISA default value will be used as a fallback.
+
 config CPU_BIG_ENDIAN
 	bool "Enable Big Endian Mode"
 	help

arch/arc/Makefile

@@ -10,8 +10,25 @@ CROSS_COMPILE := $(call cc-cross-prefix, arc-linux- arceb-linux-)
 endif
 
 cflags-y	+= -fno-common -pipe -fno-builtin -mmedium-calls -D__linux__
-cflags-$(CONFIG_ISA_ARCOMPACT)	+= -mA7
-cflags-$(CONFIG_ISA_ARCV2)	+= -mcpu=hs38
+
+tune-mcpu-def-$(CONFIG_ISA_ARCOMPACT)	:= -mA7
+tune-mcpu-def-$(CONFIG_ISA_ARCV2)	:= -mcpu=hs38
+
+ifeq ($(CONFIG_ARC_TUNE_MCPU),"")
+cflags-y				+= $(tune-mcpu-def-y)
+else
+tune-mcpu				:= $(shell echo $(CONFIG_ARC_TUNE_MCPU))
+tune-mcpu-ok 				:= $(call cc-option-yn, $(tune-mcpu))
+ifeq ($(tune-mcpu-ok),y)
+cflags-y				+= $(tune-mcpu)
+else
+# The flag provided by 'CONFIG_ARC_TUNE_MCPU' option isn't known by this compiler
+# (probably the compiler is too old). Use ISA default mcpu flag instead as a safe option.
+$(warning ** WARNING ** CONFIG_ARC_TUNE_MCPU flag '$(tune-mcpu)' is unknown, fallback to '$(tune-mcpu-def-y)')
+cflags-y				+= $(tune-mcpu-def-y)
+endif
+endif
+
 
 ifdef CONFIG_ARC_CURR_IN_REG
 # For a global register defintion, make sure it gets passed to every file