arm64: write PID in CONTEXTIDR_EL1 on ctx switch

[zachary.leaf_arm.com]

Summary

Debug and trace features such as Statistical Profiling Extension (SPE)
use the CONTEXTIDR_EL1 register to get the PID of the current process.

Add a sysctl switch to toggle writing the current PID into this register
in the thread switcher.

To make use of the feature, the following sysctl switch must be set:

sysctl machdep.pid_in_contextidr=1

Kernel code can also toggle the sysctl by writing directly to the global
var behind the sysctl arm64_pid_in_contextidr:

extern bool arm64_pid_in_contextidr;

Sponsored by: Arm Ltd

[andrew]

sys/arm64/arm64/swtch.S

59–60

This should save a load

[zachary.leaf_arm.com]

use ADRP to save a load

[jrtc27]

sys/arm64/arm64/swtch.S

60

63

Presumably ldrw is right as pid_t is negative for errors, but maybe there are cases that matter here where it can be?

[zachary.leaf_arm.com]

• Use LDRB for bool arm64_pid_in_contextidr
• Use LDRW for P_PID + add comment
• Remove \label from pid_in_contextidr macro
• Change ASSYM(PR_PID... to P_PID

[zachary.leaf_arm.com]

sys/arm64/arm64/swtch.S

63

I think ldrw is correct. I can't see that struct proc->p_pid can ever be negative, only 0 or positive.

[zachary.leaf_arm.com]

Any further comments on this @jrtc27 @andrew?