Fix invalid checks for CONTEXT_XSTATE (dotnet/coreclr#6621)

Fix invalid checks for CONTEXT_XSTATE (dotnet/coreclr#6621)

Checks for context flags containing CONTEXT_XSTATE were incorrect at two places.
The issue was that CONTEXT_XSTATE is not a single bit flag, but contains two
bits set - it is CONTEXT_AMD64 | 0x40. So testing the flag using
(contextFlags & CONTEXT_XSTATE) != 0 was always true, since context flags
on AMD64 always contain CONTEXT_AMD64 and so the `&` result is always non-zero.

Commit migrated from https://github.com/dotnet/coreclr/commit/a95820d7a3b968d25cdfd71266924b39727777e6