Added sb instruction support for ARMv9 architecture

[salvatoredipietro]

We would like to add the support for ARMv9 architecture to use sb instruction instead of isb.
Based on our micro benchmark (patch attached), sb is ~30% faster than isb (ratio=1.710:1) and the change do not seems to introduce any regression on the spin_cpu_spinwait() function (isb_spin=8740725us vs standard_spin=8739722us).

# Jemalloc build
$ make clean all ; ./autogen.sh && ./configure && make -j4

# Test on m8g.2xlarge with patch
$ make tests_stress && ./test/stress/arm_spin_bench
Running on ARM64 architecture
SB instruction is supported
1000000 iterations, isb_spin=8740725us (8740.725 ns/iter), standard_spin=5110839us (5110.839 ns/iter), time consumption ratio=1.710:1

# Test on m8g.2xlarge without patch 
1000000 iterations, isb_spin=8739722us (8739.722 ns/iter), standard_spin=8739722us (8739.722 ns/iter), time consumption ratio=1.000:1

Original post: jemalloc#2843

[lexprfuncall]

Thanks for the PR! Do you have any thoughts on the use of a higher-throughput instruction for a delay loop? Intuitively, you need to waste some time so the difference between an sb and an isb could be either a win or a lose depending on the number of iterations.

As an aside, the approach suggested by ARM involves uses a more precise delay loop to account for the differences. That might require a slightly bigger change to the spin_adaptive implementation:

https://developer.arm.com/community/arm-community-blogs/b/architectures-and-processors-blog/posts/multi-threaded-applications-arm