i#7835: Add a helper function for creating syscall instruction in tests

[bin-wang]

This PR introduces a helper function (create_test_syscall()) to reduce repeated code for creating a simple syscall instruction in unit tests. I created new test_helpers_syscall.h/cpp to put this helper function in.

Alternatives considered:

1. Put the declaration and definition of this help function in existing test_helpers.h/cpp. This way we don't need to add new files, however there are 2 drawbacks: 1). test_helpers (and therefore tests that use test_helpers) now need to be linked against drdecode. 2) We probably need to change the unsupported architecture error from a compile time error to runtime error, because we want be able to compile other tests on those architectures.

2. Declare the helper function in test_helpers.h, define it in test_helpers_syscall.cpp. This is pretty much the same as what's currently implemented, just one less header file.

Fixes #7835

[derekbruening]

OP_syscall is not supported on 32-bit Intel so this won't work for a test that needs to execute this instruction, which should be documented for a shared function. But, better to use sysenter I would think.

[derekbruening]

IMHO we should just add XINST_CREATE_syscall in the ir instr_create_api files, with comments clearly explaining what it does and what its limitations are (maybe hardcode as sysenter for x86_32 define and syscall for x86_64 (though we want to get away from ifdefs in the IR for better cross-arch support: #1684); maybe no WOW64 support). That should help multiple other use cases outside of drmemtrace as well.