BL31: Enable pointer authentication support

The size increase after enabling options related to ARMv8.3-PAuth is:

+----------------------------+-------+-------+-------+--------+
|                            |  text |  bss  |  data | rodata |
+----------------------------+-------+-------+-------+--------+
| CTX_INCLUDE_PAUTH_REGS = 1 |  +192 | +1536 |   +0  |   +0   |
|                            |  0.3% |  3.1% |       |        |
+----------------------------+-------+-------+-------+--------+
| ENABLE_PAUTH = 1           | +1848 | +1536 |  +16  |   +0   |
|                            |  3.3% |  3.1% |  3.1% |        |
+----------------------------+-------+-------+-------+--------+

Results calculated with the following build configuration:

    make PLAT=fvp SPD=tspd DEBUG=1 \
    SDEI_SUPPORT=1                 \
    EL3_EXCEPTION_HANDLING=1       \
    TSP_NS_INTR_ASYNC_PREEMPT=1    \
    CTX_INCLUDE_PAUTH_REGS=1       \
    ENABLE_PAUTH=1

Change-Id: I43db7e509a4f39da6599ec2faa690d197573ec1b
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
diff --git a/bl31/bl31.mk b/bl31/bl31.mk
index c9ba926..10feae1 100644
--- a/bl31/bl31.mk
+++ b/bl31/bl31.mk
@@ -75,6 +75,10 @@
 BL31_SOURCES		+=	lib/extensions/mpam/mpam.c
 endif
 
+ifeq (${ENABLE_PAUTH},1)
+BL31_CFLAGS		+=	-msign-return-address=non-leaf
+endif
+
 ifeq (${WORKAROUND_CVE_2017_5715},1)
 BL31_SOURCES		+=	lib/cpus/aarch64/wa_cve_2017_5715_bpiall.S	\
 				lib/cpus/aarch64/wa_cve_2017_5715_mmu.S