arm64: probes: Fix incorrect bl/blr address and register usage

The pt_regs registers are 64-bit on arm64, and should be u64 when
manipulated. Correct this so that we aren't truncating the address
during br/blr sequences.

Fixes: efb07ac5 ("arm64: probes: Add GCS support to bl/blr/ret")
Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>