x86: Stop building the vboot library using regparm=3.

This complicates things in a number of ways, including making GDB not work
properly because it assumes the standard ABI in some places. Measurements
show that it doesn't really make much difference performance wise.

TEST=Built and booted with coreboot and depthcharge on link.

Change-Id: I7f004f8cf83b7c1a78ab12f814477504a5a5c28c
Signed-off-by: Gabe Black <gabeblack@google.com>
Reviewed-on: https://chromium-review.googlesource.com/183988
Reviewed-by: Shawn Nematbakhsh <shawnn@chromium.org>
Commit-Queue: Shawn Nematbakhsh <shawnn@chromium.org>
Tested-by: Shawn Nematbakhsh <shawnn@chromium.org>
diff --git a/Makefile b/Makefile
index d4b6245..ffe3946 100644
--- a/Makefile
+++ b/Makefile
@@ -132,7 +132,7 @@
 CFLAGS ?= \
 	-ffunction-sections -fvisibility=hidden -fno-strict-aliasing \
 	-fomit-frame-pointer -fno-toplevel-reorder -fno-dwarf2-cfi-asm \
-	-mpreferred-stack-boundary=2 -mregparm=3 \
+	-mpreferred-stack-boundary=2 \
 else ifeq (${FIRMWARE_ARCH}, x86_64)