arm64: Add unwind information to _start

[dchagin]

Summary

The right unwinding stop indicator should be CFI-undefined PC.
http://dwarf.freestandards.org/Dwarf3.pdf - page 118:

• If a Return Address register is defined in the virtual unwind table,

and its rule is undefined (for example, by DW_CFA_undefined), then
there is no return address and no call address, and the virtual
unwind of stack activations is complete.

Create an initial frame with 0 lr and fp.
Procedure Call Standard for the ARM 64-bit Architecture (AArch64)
specifies that the end of the frame record chain is indicated by
the address zero in the address for the previous frame.

This is necessary to complete fixing the libunwind for Aarch64.

MFC after: 1 month

[dchagin]

I plan to commit this soon, any objections?

[imp]

I thought the kernel zeroed these... but I have no objections since my aarch64 fu is weak

[dchagin]

In D40277#924038, @imp wrote:

I thought the kernel zeroed these... but I have no objections since my aarch64 fu is weak

For the dynamically linked binaries there is no other proper way to tell the end frame to the unwinder, other than bad heuristic guessing games.
_start is the entry point for binary, which is called by rtld in that case.

[emaste]

lib/csu/aarch64/crt1_s.S

37–38

whitespace doesn't match it seems -- tabs vs spaces?

[dchagin]

lib/csu/aarch64/crt1_s.S

37–38

Hmm, looks like a phabricator rendering problems, the patch contains tabs.
Second argument is just arranged to the second arguments

[dchagin]

https://reviews.freebsd.org/D40623