macOS Catalina: FAIL: t-powm

[Jack Howarth]

FYI, I reported this issue to Apple via radar and their response was...

Engineering has provided the following information regarding this issue:
It sounds like gmp has a memory corruption issue which -fstack-check has
uncovered.  We suggest filing a bug with the developers of gmp.

So they believe clang is accurately detecting a bug in gmp. A back trace of
the failure shows...

bash-3.2# lldb ./t-powm
lldb ./t-powm
(lldb) target create "./t-powm"
Current executable set to './t-powm' (x86_64).
(lldb) r
Process 39643 launched:
'/Users/howarth/gmp_regression/default_build/t-powm' (x86_64)
Process 39643 stopped
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS
(code=EXC_I386_GPFLT)
    frame #0: 0x00007fff71afc386
libdyld.dylib`stack_not_16_byte_aligned_error
libdyld.dylib`stack_not_16_byte_aligned_error:
->  0x7fff71afc386 <+0>: movdqa %xmm0, (%rsp)
    0x7fff71afc38b <+5>: int3
    0x7fff71afc38c <+6>: nop
    0x7fff71afc38d <+7>: nop
Target 0: (t-powm) stopped.
(lldb) bt
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS
(code=EXC_I386_GPFLT)
  * frame #0: 0x00007fff71afc386
libdyld.dylib`stack_not_16_byte_aligned_error

I am attaching the test case I created for them. Note that the failure can
be triggered with only the t-powm executable built using the default stack
checking so the problem seems to lie there and not in the gmp shared
libraries.
              Jack
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