runtime: use new #include "textflag.h"

I did this just to clean things up, but it will be important
when we drop the pkg directory later.

LGTM=bradfitz
R=r, bradfitz
CC=golang-codereviews
https://codereview.appspot.com/132600043
syscall: in linux/arm Syscall, zero R3, R4, R5

The general kernel system call interface
takes 6 arguments: R0, R1, R2, R3, R4, R5.

Syscall is for calls that only need 3.
The amd64 and 386 versions zero the extra arg registers,
but the arm version does not.

func utimensat calls Syscall with 3 arguments.
The kernel expects a 4th argument.
That turns out to be whatever is in R3 at the time of the call.
CL 137160043 changed various pieces of code and apparently
changed the value left in R3 at the time of utimensat's Syscall.
This causes the kernel to return EINVAL.

Change linux/arm Syscall to zero R3, R4, R5, so that calls will
behave deterministically, even if they pass too few arguments.

Arguably, utimensat could be fixed too, but the predictable
zeroing is certainly worth doing, and once done utimensat's
use of Syscall is fine.

Fixes arm build.

TBR=bradfitz
CC=golang-codereviews
https://codereview.appspot.com/141080043
runtime: use cas loop to coordinate with sigprof

sigprof and setcpuprofilerate coordinate the enabling/disabling
of the handler using a Mutex. This has always been a bit dodgy:
setcpuprofilerate must be careful to turn off signals before acquiring
the lock to avoid a deadlock.

Now the lock implementations use onM, and onM isn't okay on the
signal stack. We know how to make it okay, but it's more work than
is probably worth doing.

Since this is super-dodgy anyway, replace the lock with a simple
cas loop. It is only contended if setcpuprofilerate is being called,
and that doesn't happen frequently enough to care about the
raw speed or about using futexes/semaphores.

TBR to fix freebsd/amd64 and dragonfly/amd64 builds.
Happy to make changes in a follow-up CL.

TBR=dvyukov
CC=golang-codereviews
https://codereview.appspot.com/141080044