lib: Update ASan compiler flags and Kconfig help text

An additional compiler flag is added to make use of the shadow
offset callback feature we introduced in our GCC patch. Also,
a comment is added to tell user that this GCC patch needs to be
applied in order to use ASan.

Change-Id: Ia187e4991bf808f4ae137eff0ffdb9baea0085e9
Signed-off-by: Harshit Sharma <harshitsharmajs@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43164
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Werner Zeh <werner.zeh@siemens.com>

src/lib/Makefile.inc

@@ -9,7 +9,8 @@ endif
 
 ifeq ($(CONFIG_ASAN_IN_RAMSTAGE),y)
 ramstage-y += asan.c
-CFLAGS_asan += -fsanitize=kernel-address \
+# Ensure that asan_shadow_offset_callback patch is applied to GCC before ASan is used.
+CFLAGS_asan += -fsanitize=kernel-address --param asan-use-shadow-offset-callback=1 \
 		--param asan-stack=1 --param asan-globals=1 \
 		--param asan-instrumentation-with-call-threshold=0 \
 		-fsanitize-address-use-after-scope \