Enable arm64 optimizations that exist for power/x86 (#3393)

* Enable unaligned accesses on arm64

64-bit Arm platforms support unaligned accesses.

Running the string benchmarks this change improves performance
by an average of 1.04x, min .96x, max 1.21x, median 1.01x

* arm64 enable gc optimizations

Similar to x86 and powerpc optimizations.

|       |compare-ruby|built-ruby|
|:------|-----------:|---------:|
|hash1  |       0.225|     0.237|
|       |           -|     1.05x|
|hash2  |       0.110|     0.110|
|       |       1.00x|         -|

* vm_exec.c: improve performance for arm64

|                               |compare-ruby|built-ruby|
|:------------------------------|-----------:|---------:|
|vm_array                       |     26.501M|   27.959M|
|                               |           -|     1.06x|
|vm_attr_ivar                   |     21.606M|   31.429M|
|                               |           -|     1.45x|
|vm_attr_ivar_set               |     21.178M|   26.113M|
|                               |           -|     1.23x|
|vm_backtrace                   |       6.621|     6.668|
|                               |           -|     1.01x|
|vm_bigarray                    |     26.205M|   29.958M|
|                               |           -|     1.14x|
|vm_bighash                     |    504.155k|  479.306k|
|                               |       1.05x|         -|
|vm_block                       |     16.692M|   21.315M|
|                               |           -|     1.28x|
|block_handler_type_iseq        |       5.083|     7.004|
|                               |           -|     1.38x|

1 files changed, 1 insertions, 1 deletions

diff --git a/siphash.c b/siphash.c
index 153d2c690a..ddf8ee245d 100644
--- a/siphash.c
+++ b/siphash.c
@@ -30,7 +30,7 @@
 #ifndef UNALIGNED_WORD_ACCESS
 # if defined(__i386) || defined(__i386__) || defined(_M_IX86) || \
      defined(__x86_64) || defined(__x86_64__) || defined(_M_AMD64) || \
-     defined(__powerpc64__) || \
+     defined(__powerpc64__) || defined(__aarch64__) || \
      defined(__mc68020__)
 #   define UNALIGNED_WORD_ACCESS 1
 # endif