crt0 cleanup

- use the generic 'crt0.s' for Linux
- move the read-only '__dso_handle' definition into the '.text' section
- move the '__initial_sp' definition into the '.bss' section
- remove the '_main_utcb' definition

Part of #766.

base/lib/mk/startup.inc

@@ -3,4 +3,6 @@ SRC_CC  += _main.cc
 
 REP_INC_DIR += src/platform
 
+LIBS += syscall
+
 vpath _main.cc $(BASE_DIR)/src/platform

base/src/platform/arm/crt0.s

@@ -26,15 +26,8 @@ _start:
 .initial_sp: .word __initial_sp
 .stack_high: .word _stack_high
 
-/*--------------------------------------------------*/
-        .data
 	.globl	__dso_handle
-__dso_handle:
-	.long	0
-
-	.globl  __initial_sp
-__initial_sp:
-	.long   0
+__dso_handle: .long 0
 
 /*--- .bss (non-initialized data) ------------------*/
 .section ".bss"
@@ -46,10 +39,6 @@ _stack_low:
 	.globl	_stack_high
 _stack_high:
 
-	/*
-	 * Symbol referenced by ldso's crt0.s, which is needed by base-hw only.
-	 * It is defined here merely to resolve the symbol for non-base-hw
-	 * platforms.
-	 */
-	 .globl _main_utcb
-	_main_utcb: .long 0
+	/* initial value of the SP register */
+	.globl  __initial_sp
+__initial_sp: .space 4

base/src/platform/x86_32/crt0.s

@@ -33,16 +33,8 @@ _start:
 	.ascii "_main() returned."
 2:	jmp  1b
 
-
-/*--------------------------------------------------*/
-	.data
 	.globl	__dso_handle
-__dso_handle:
-	.long	0
-
-	.globl	__initial_sp
-__initial_sp:
-	.long	0
+__dso_handle: .long 0
 
 /*--- .eh_frame (exception frames) -----------------*/
 /*
@@ -59,3 +51,7 @@ _stack_low:
 	.space	64*1024
 	.global	_stack_high
 _stack_high:
+
+	/* initial value of the ESP register */
+	.globl	__initial_sp
+__initial_sp: .space 4

base/src/platform/x86_64/crt0.s

@@ -36,17 +36,8 @@ _start:
 	.ascii "_main() returned."
 2:	jmp  1b
 
-
-/*--------------------------------------------------*/
-	.data
-	.p2align 8
 	.globl	__dso_handle
-__dso_handle:
-	.quad	0
-
-	.globl	__initial_sp
-__initial_sp:
-	.quad	0
+__dso_handle: .quad 0
 
 /*--- .eh_frame (exception frames) -----------------*/
 /*
@@ -63,3 +54,7 @@ _stack_low:
 	.space	64*1024
 	.global	_stack_high
 _stack_high:
+
+	/* initial value of the RSP register */
+	.globl	__initial_sp
+__initial_sp: .space 8