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 <broonie@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>