update gcc linker script: fixed __bss_start align error.

bsp/beaglebone/beaglebone_ram.lds

@@ -67,7 +67,7 @@ SECTIONS
     __data_end = .;
 
     . = ALIGN(4);
-    __bss_start = __data_end;
+    __bss_start = .;
     .bss       :
     {
     *(.bss)

bsp/imx6sx/cortex-a9/imx6.lds

@@ -75,7 +75,7 @@ SECTIONS
     __data_end = .;
 
     . = ALIGN(4);
-    __bss_start = __data_end;
+    __bss_start = .;
     .bss       :
     {
     *(.bss)

bsp/zynq7000/zynq7000.ld

@@ -92,7 +92,7 @@ SECTIONS
     __data_end = .;
 
     . = ALIGN(4);
-    __bss_start = __data_end;
+    __bss_start = 0;
     .bss       :
     {
     *(.bss)