boards: sabre-6quad: Fix dramboot.ld for libcxx

Summary:
- I noticed that cxxtest does not work correctly.
- Finally, I found that initializers for c++ is not called.
- This commit fixes this issue

Impact:
- None

Testing:
- Tested with cxxtest (defconfig will be added later)

Signed-off-by: Masayuki Ishikawa <Masayuki.Ishikawa@jp.sony.com>

boards/arm/imx6/sabre-6quad/scripts/dramboot.ld

@@ -60,7 +60,7 @@ SECTIONS
     .init_section :
     {
         _sinit = ABSOLUTE(.);
-        *(.init_array .init_array.*)
+        KEEP(*(.init_array .init_array.*))
         _einit = ABSOLUTE(.);
     } > ddr3