[SYCL] tighten known_identity to finite_math macro

[cperkinsintel]

Some time ago we fixed a bug in sycl::known_identity where it was incorrectly using std::numeric_limits<T>::infinity() even in cases where ::infinity() should not have been available. At the time, the fix was to simply test for __FAST_MATH__.

That worked, but was not ideal. Now we are tightening this to use the __FINITE_MATH_ONLY__ macro which is more accurate.
see #13813

__FINITE_MATH_ONLY__ will be set when both the -fno-honor-infinities and -fno-honor-nans flags are used. It was felt by the FE team that there is no reasonable case where one of the flags would be used without the other and therefore __FINITE_MATH_ONLY__ is sufficient for this problem ( and superior to the __FAST_MATH__ macro we were using before).

[aelovikov-intel]

see CMPLRLLVM-58728, CMPLRLLVM-65507

This isn't approved usage for referencing internal bug reports. We are only supposed to do that when we can't share information there, e.g. issues in underlying proprietary layers. Anything concerning SYCL RT or C/C++ FE is public and should be accessible for anybody reading this PR/commit message.

[cperkinsintel]

@aelovikov-intel - I encouraged everyone to use issue #13813 for technical discussions, but some of it was conducted on the JIRAS. This particular PR is pretty simple, I don't have anything to add to the description or the comment.

[aelovikov-intel]

Please summarize relevant bits from these internal bug-reports directly in the PR description and remove references to the jiras.