Don't overflow the trap frame when accessing lr or xzr

[andrew]

Summary

When emulating a load pair or store pair in dtrace on arm64 we need to
copy the data between the stack and trap frame. When the registers are
either the link register or the zero register we will access memory
past the end of the trap frame as these are encoded as registers 30 and
31 respectively while the array they access only has 30 entries.

Fix this by creating 2 helper functions to perform the operation with
special cases for these registers.

Test Plan

Can run the following without a kernel panic:
dtrace -n 'fbt::kmem*:entry { @[probefunc] = count(); }'

[andrew]

Committed in rS363272