arm64: Disable per-thread stack-smashing protection in data_abort()

With PERTHREAD_SSP configured, the compiler's stack-smashing protection
uses a per-thread canary value instead of a global value. The value is
stored in td->td_md.md_canary; the sp_el0 register always contains a
pointer to that value, and certain functions selected by the compiler
will store the canary value on the stack as a part of the function
prologue (and will verify the copy as part of the epilogue). In
particular, the thread structure may be accessed.

This happens to occur in data_abort(), which leads to the same problem
addressed by commit 2c10be9e06d4 ("arm64: Handle translation faults for
thread structures"). This commit fixes that directly, by disabling SSP
in data_abort() and a couple of related functions by using a function
attribute. It also moves the update of sp_el0 out of C code in case
the compiler decides to start checking the canary in pmap_switch()
someday.

A different solution might be to move the canary value to the PCB, which
currently lives on the kernel stack and isn't subject to the same
problem as thread structures (if only because guard pages inhibit
superpage promotion). However, there isn't any particular reason the
PCB has to live on the stack today; on amd64 it is embedded in struct
thread, reintroducing the same problem. Keeping the reference canary
value at the top of the stack is also rather dubious since it could be
clobbered by a sufficiently large stack overflow.

A third solution could be to go back to the approach of commit
5aa5420ff2e8, and modify UMA to use the direct map for thread structures
even if KASAN is enabled. But, transient promotions and demotions in
the direct map are possible too.

Reviewed by: alc, kib, andrew
MFC after: 1 month
Sponsored by: Juniper Networks, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D37255