runtime/pprof: prevent a deadlock that SIGPROF might create on mips{,le}

64bit atomics on mips/mipsle are implemented using spinlocks. If SIGPROF
is received while the program is in the critical section, it will try to
write the sample using the same spinlock, creating a deadloop.
Prevent it by creating a counter of SIGPROFs during atomic64 and
postpone writing the sample(s) until called from elsewhere, with
pc set to _LostSIGPROFDuringAtomic64.

Added a test case, per Cherry's suggestion. Works around #20146.

Change-Id: Icff504180bae4ee83d78b19c0d9d6a80097087f9
Reviewed-on: https://go-review.googlesource.com/42652
Run-TryBot: Cherry Zhang <cherryyz@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

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