Add proper C runtime init, add reset handler.

The C runtime wasn't initialized correctly (there was garbage in the data
and bss sections). Add a reset_handler which initializes these sections
before calling the application's main() function.

The initial stack pointer is also defined in the linker script, allowing the
application to override with a linker command line option
"-Wl,--defsym,_stack=0x20005000".

Thanks to Gareth McMullin <gareth@blacksphere.co.nz>.

lib/libopenstm32.ld

@@ -33,15 +33,25 @@ SECTIONS
 		*(.vectors)	/* Vector table */
 		*(.text)	/* Program code */
 		*(.rodata)	/* Read-only data */
+		_etext = .;
 	} >rom
 
 	. = ORIGIN(ram);
 
 	.data : {
+		_data = .;
 		*(.data)	/* Read-write initialized data */
+		_edata = .;
 	} >ram AT >rom
 
 	.bss : {
 		*(.bss)		/* Read-write zero initialized data */
+		*(COMMON)
+		_ebss = .;
 	} >ram AT >rom
+
+	end = .;
 }
+
+PROVIDE(_stack = 0x20000800);
+