sys: Build arm64 per-thread SSP with GCC

It has been supported since GCC 9. It is unlikely anything older than
that will build the kernel so mark it as supported by GCC.

Reviewed by:	brooks, jhb
Sponsored by:	Arm Ltd
Differential Revision:	https://reviews.freebsd.org/D45267

sys/conf/Makefile.arm64

@@ -30,7 +30,8 @@ CFLAGS += -DLINUX_DTS_VERSION=\"${LINUX_DTS_VERSION}\"
 
 PERTHREAD_SSP_ENABLED!=	grep PERTHREAD_SSP opt_global.h || true ; echo
 .if !empty(PERTHREAD_SSP_ENABLED)
-. if ${COMPILER_TYPE} == "clang" && ${COMPILER_VERSION} >= 130000
+. if (${COMPILER_TYPE} == "clang" && ${COMPILER_VERSION} >= 130000) || \
+  ${COMPILER_TYPE} == "gcc"
 ARM64_SSP_CFLAGS = -mstack-protector-guard=sysreg
 ARM64_SSP_CFLAGS += -mstack-protector-guard-reg=sp_el0
 ARM64_SSP_CFLAGS += -mstack-protector-guard-offset=0