xtensa: abstract 'entry' and 'retw' in assembly code

Provide abi_entry, abi_entry_default, abi_ret and abi_ret_default macros
that allocate aligned stack frame in windowed and call0 ABIs.
Provide XTENSA_SPILL_STACK_RESERVE macro that specifies required stack
frame size when register spilling is involved.
Replace all uses of 'entry' and 'retw' with the above macros.
This makes most of the xtensa assembly code ready for XEA3 and call0 ABI.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>

arch/xtensa/lib/memset.S

@@ -34,7 +34,7 @@
 ENTRY(__memset)
 WEAK(memset)
 
-	entry	sp, 16		# minimal stack frame
+	abi_entry_default
 	# a2/ dst, a3/ c, a4/ length
 	extui	a3, a3, 0, 8	# mask to just 8 bits
 	slli	a7, a3, 8	# duplicate character in all bytes of word
@@ -48,7 +48,7 @@ WEAK(memset)
 	srli	a7, a4, 4	# number of loop iterations with 16B
 				# per iteration
 	bnez	a4, .Laligned
-	retw
+	abi_ret_default
 
 /*
  * Destination is word-aligned.
@@ -95,7 +95,7 @@ EX(10f) s16i	a3, a5,  0
 EX(10f) s8i	a3, a5,  0
 .L5:
 .Lret1:
-	retw
+	abi_ret_default
 
 /*
  * Destination is unaligned
@@ -139,7 +139,7 @@ EX(10f) s8i	a3, a5, 0
 	blt	a5, a6, .Lbyteloop
 #endif /* !XCHAL_HAVE_LOOPS */
 .Lbytesetdone:
-	retw
+	abi_ret_default
 
 ENDPROC(__memset)
 
@@ -150,4 +150,4 @@ ENDPROC(__memset)
 
 10:
 	movi	a2, 0
-	retw
+	abi_ret_default