arm64: kconfig: Update and comment GCC version check for kernel BTI

Some versions of GCC are known to suffer from a BTI code generation bug,
meaning that CONFIG_CC_HAS_BRANCH_PROT_PAC_RET_BTI cannot be solely used
to determine whether or not we can compile with kernel with BTI enabled.

Update the BTI Kconfig entry to refer to the relevant GCC bugzilla entry
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94697) and update the check
now that the fix has been merged into GCC release 10.1.

Acked-by: Mark Brown <[email protected]>
Signed-off-by: Will Deacon <[email protected]>

Author: willdeacon

arch/arm64/Kconfig

@@ -1616,7 +1616,8 @@ config ARM64_BTI_KERNEL
 	depends on ARM64_BTI
 	depends on ARM64_PTR_AUTH
 	depends on CC_HAS_BRANCH_PROT_PAC_RET_BTI
-	depends on !CC_IS_GCC || GCC_VERSION >= 100000
+	# https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94697
+	depends on !CC_IS_GCC || GCC_VERSION >= 100100
 	depends on !(CC_IS_CLANG && GCOV_KERNEL)
 	depends on (!FUNCTION_GRAPH_TRACER || DYNAMIC_FTRACE_WITH_REGS)
 	help