Implement PAL_IsDebuggerPresent for SunOS

[am11]

On illumos and Solaris, unlike Linux, /proc/{pid}/status does not:

• return a textual representation
• contain TracerPID: when debugger is attached

Instead it returns data for struct pstatus_t.

When the debugger is attached, the flags are set in fltset_t pr_flttrace field, where fltset_t is defined as a struct wrapping int word[4] and it is described in proc(4) as /* set of traced faults */.

I have tested it under the debugger and this (zero'th) index is always set.

---

Also:

• fix some fatal warnings (as-error) in palsuite, reported by gcc 8.4 in illumos sysroot.
• fix some non-fatal warnings in JIT, reported by gcc 8 and also in CI, with gcc 7.

Contributes to: #34944.

[am11]

Some of the remaining gcc warnings have explanation in code:

src/coreclr/src/jit/codegenxarch.cpp:2510:54: warning: comparison of integer expressions of different signedness: 'int' and 'size_t' {aka 'long unsigned int'} [-Wsign-compare]
src/coreclr/src/jit/codegenxarch.cpp:83:54: warning: comparison of integer expressions of different signedness: 'int' and 'GSCookie' {aka 'long unsigned int'} [-Wsign-compare]
src/coreclr/src/jit/emitxarch.cpp:7069:89: warning: comparison of integer expressions of different signedness: 'int' and 'size_t' {aka 'long unsigned int'} [-Wsign-compare]
src/coreclr/src/jit/emitxarch.cpp:7105:77: warning: comparison of integer expressions of different signedness: 'int' and 'size_t' {aka 'long unsigned int'} [-Wsign-compare]

and all the rest are related to #33541.

[am11]

cc @janvorli, @jclulow

[janvorli]

The PAL and PAL tests changes look good.
@dotnet/jit-contrib can someone please review the jit changes?

[BruceForstall]

One minor request

[BruceForstall]

@am11 can you push an update (e.g., rebase to master) to force the tests to run again, and hope they don't have infrastructure failures?

[am11]

@BruceForstall, rebased to master.