coreboot: x86: enable gc-sections

Garbage collected sections allow for trimming the size of the
binaries as well as allowing for not needing to config off
unused functions. To that end, on a rambi build the following
differences are observed:

$ diff -up \
	<(readelf -l coreboot-builds/google_rambi/cbfs/fallback/ramstage.elf) \
	<(readelf -l coreboot-builds/google_rambi_gc_sections/cbfs/fallback/ramstage.elf)
--- /dev/fd/63  2015-03-10 12:07:27.927985430 -0500
+++ /dev/fd/62  2015-03-10 12:07:27.927985430 -0500
@@ -6,9 +6,9 @@ There are 4 program headers, starting at
 Program Headers:
   Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg
Align
   LOAD           0x001000 0x00000000 0x00000000 0x00040 0x00040 RWE 0
-  LOAD           0x001040 0x00000040 0x00000040 0x34560 0x34560 RWE 0
-  LOAD           0x0355a0 0x000345a0 0x000345a0 0x02578 0x02578 RWE 0
-  LOAD           0x037b18 0x00036b18 0x00036b18 0x00000 0x0b560     0
+  LOAD           0x001040 0x00000040 0x00000040 0x2cbf8 0x2cbf8 RWE 0
+  LOAD           0x02dc38 0x0002cc38 0x0002cc38 0x02208 0x02208 RWE 0
+  LOAD           0x02fe40 0x0002ee40 0x0002ee40 0x00000 0x0a888     0

  Section to Segment mapping:
   Segment Sections...

$ diff -up \
	<(readelf -l coreboot-builds/google_rambi/cbfs/fallback/romstage.elf) \
	<(readelf -l coreboot-builds/google_rambi_gc_sections/cbfs/fallback/romstage.elf)
--- /dev/fd/63  2015-03-10 12:08:16.855985880 -0500
+++ /dev/fd/62  2015-03-10 12:08:16.851985880 -0500
@@ -5,8 +5,8 @@ There are 1 program headers, starting at

 Program Headers:
   Type           Offset   VirtAddr   PhysAddr   FileSiz MemSiz  Flg
Align
-  LOAD           0x000060 0xfff20000 0xfff20000 0x08b81 0x08b81 R E
   0x10
+  LOAD           0x000060 0xfff20000 0xfff20000 0x06300 0x06300 R E
0x10

  Section to Segment mapping:
   Segment Sections...
-   00     .rom .text
+   00     .rom

The following warnings needed to be applied to CFLAGS_common because for
some reason gcc was miraculously emitting the warnings with the
unrelated *-sections options:
  -Wno-unused-but-set-variable

Change-Id: I210784fdfc273ce4cb9927352cbd5a51be3c6929
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: http://review.coreboot.org/8635
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>

src/lib/Makefile.inc

@@ -119,7 +119,7 @@ ramstage-y += rmodule.c
 romstage-$(CONFIG_RELOCATABLE_RAMSTAGE) += rmodule.c
 
 RMODULE_LDSCRIPT := $(src)/lib/rmodule.ld
-RMODULE_LDFLAGS  := -nostartfiles --emit-relocs -z defs -Bsymbolic -T$(RMODULE_LDSCRIPT)
+RMODULE_LDFLAGS  := -nostartfiles --gc-sections --emit-relocs -z defs -Bsymbolic -T$(RMODULE_LDSCRIPT)
 
 # rmodule_link_rules is a function that should be called with:
 # (1) the object name to link