i#5383 mac a64, part 6: Get OSX-labeled tests to succeed

[derekbruening]

8 of the 13 tests on Mac AArch64 labeled "OSX" fail prior to this PR. Here we fix the following:

• Syscall success is indicated by the carry flag just like x86 Mac
• Handle sigreturn with its extra parameters just like x86 Mac
• Fix signal handler parameters
• Fix stolen register support in signal contexts
• Use MAP_JIT and pthread_jit_write_protect_np for +rwx gencode in tests
• Use DYLD_LIBRARY_PATH on Mac in tests

Now all 13 tests pass:

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$ ctest -j 5 -L OSX
 1/13 Test  #13: code_api|common.fib ................................  Passed  0.59 sec
 2/13 Test #243: code_api|libutil.frontend_test .....................  Passed  0.63 sec
 3/13 Test #231: code_api|api.ir ....................................  Passed  0.67 sec
 4/13 Test   #9: code_api|linux.sigaction.native ....................  Passed  0.25 sec
 5/13 Test  #31: code_api|linux.signal0000 ..........................  Passed  0.10 sec
 6/13 Test #240: code_api|api.ir-static .............................  Passed  0.34 sec
 7/13 Test #241: code_api|api.drdecode ..............................  Passed  0.38 sec
 8/13 Test #245: code_api|api.dis-a64 ...............................  Passed  1.15 sec
 9/13 Test #264: no_code_api,no_intercept_all_signals|linux.sigaction  Passed  0.08 sec
10/13 Test  #33: code_api|linux.signal0010 ..........................  Passed  0.34 sec
11/13 Test  #35: code_api|linux.signal0100 ..........................  Passed  0.42 sec
12/13 Test  #37: code_api|linux.signal0110 ..........................  Passed  0.45 sec
13/13 Test   #7: samples_proj .......................................  Passed  1.89 sec
100% tests passed, 0 tests failed out of 13
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Issue: #5383