sdt: Disable SDT probes in kernel modules for GCC on aarch64

[jhb]

Summary

For PIC code, the existing assembly constraints do compile on aarch64.
Some kernel modules build ok using the 'p' constraint with the 'a'
operand modifier, but not all.

[jhb]

I do not like this at all, but I can't find a way to get this to work with GCC on arm64. I tried various combinations of constraints and modifiers to no avail.

[jhb]

FYI, this stack of changes lets me build aarch64 world + GENERIC with GCC 14 and 15.

[markj]

Some kernel modules build ok using the 'p' constraint with the 'a' operand modifier, but not all.

Hmm, that seems a bit surprising. What goes wrong? I wonder if it works to try declaring a local variable struct sdt_probe *p = &_SDT_PROBE_NAME(...) and passing that as an operand instead.

[jhb]

In D55166#1261396, @markj wrote:

Some kernel modules build ok using the 'p' constraint with the 'a' operand modifier, but not all.

Hmm, that seems a bit surprising. What goes wrong? I wonder if it works to try declaring a local variable struct sdt_probe *p = &_SDT_PROBE_NAME(...) and passing that as an operand instead.

I can try to reproduce, but I ended up with :lo12:LADDR0 or the like as the resulting argument to .word in the assembly in the case it breaks.