feat(lib/stack_protector): Add stack protector option

This patch adds `-fstack-protector-strong` as a USER
build option to TF-RMM.

Fixes #35

Signed-off-by: Jacob Man Chun Yiu <jacobmanchun.yiu@arm.com>
Change-Id: I5cd9898a4433d445b7a701288040f7b7476f28d3
diff --git a/docs/getting_started/build-options.rst b/docs/getting_started/build-options.rst
index 7c434ad..0fa4a77 100644
--- a/docs/getting_started/build-options.rst
+++ b/docs/getting_started/build-options.rst
@@ -297,6 +297,7 @@
    ATTEST_PLAT_TOKEN_SIZE	,			,0x1000			,"Maximum size in bytes expected for the Attestation platform token"
    PLAT_ARM_MAX_MEM_BANKS	,			,2			,"Maximum possible number of DRAM and COH/NCOH device memory banks allowed in Arm platform layer"
    ATTEST_EL3_TOKEN_SIGN	,ON|OFF			,OFF			,"Use EL3 service to sign realm attestation token."
+   STACK_PROTECTOR		,ON | OFF		,OFF			,"Enable the stack protector compiler option."
 
 .. _llvm_build: