arm64: consistently use bl for C exception entry

In most cases, our exception entry assembly branches to C handlers with
a BL instruction, but in cases where we do not expect to return, we use
B instead.

While this is correct today, it means that backtraces for fatal
exceptions miss the entry assembly (as the LR is stale at the point we
call C code), while non-fatal exceptions have the entry assembly in the
LR. In subsequent patches, we will need the LR to be set in these cases
in order to backtrace reliably.

This patch updates these sites to use a BL, ensuring consistency, and
preparing for backtrace rework. An ASM_BUG() is added after each of
these new BLs, which both catches unexpected returns, and ensures that
the LR value doesn't point to another function label.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Will Deacon <will.deacon@arm.com>