Fix FC_NO_TAILCALL with newer compilers

[jkotas]

The C11 standard treats trivial infinite loops as undefined behavior and allows
compilers to optimize them out completely.

Add "volatile" on FC_NO_TAILCALL to defeat this optimization

[Dotnet-GitSync-Bot]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[jkotas]

@trylek This should fix the intermittent crash that you have seen on x86

[jkotas]

From C standard, 6.8.5:
An iteration statement whose controlling expression is not a constant expression, that performs no input/output operations, does not access volatile objects, and performs no synchronization or atomic operations in its body, controlling expression, or (in the case of a for statement) its expression-3, may be assumed by the implementation to terminate.

[jkotas]

coreclr!ObjectNative::GetClass on Win x86

Before this change:

...
6210ec3d 8b4614          mov     eax,dword ptr [esi+14h]
6210ec40 8b401c          mov     eax,dword ptr [eax+1Ch]
6210ec43 8b4024          mov     eax,dword ptr [eax+24h]
6210ec46 85c9            test    ecx,ecx
6210ec48 7508            jne     coreclr!ObjectNative::GetClass+0x32 (6210ec52)
6210ec4a 8bca            mov     ecx,edx
6210ec4c 5e              pop     esi
6210ec4d e917000000      jmp     coreclr!GetClassHelper (6210ec69) <- This is bug. It cannot be a tailcall for the helper frame unwinder to work
...

After this change (matches what shipped in .NET 5):

...
7990018d 8b4614          mov     eax,dword ptr [esi+14h]
79900190 8b401c          mov     eax,dword ptr [eax+1Ch]
79900193 8b4024          mov     eax,dword ptr [eax+24h]
79900196 85c9            test    ecx,ecx
79900198 7512            jne     CoreCLR!ObjectNative::GetClass+0x3c (799001ac)
7990019a 8bca            mov     ecx,edx
7990019c e822000000      call    CoreCLR!GetClassHelper (799001c3) <- No tail call
799001a1 833d9477be7900  cmp     dword ptr [CoreCLR!FC_NO_TAILCALL (79be7794)],0
799001a8 75e1            jne     CoreCLR!ObjectNative::GetClass+0x1b (7990018b)
799001aa ebf5            jmp     CoreCLR!ObjectNative::GetClass+0x31 (799001a1)
...

[jkotas]

cc @hoyosjs . This is fixing a different problem, but it is in the same area as #45906

[trylek]

LGTM, thank you!