clarify printf asterisk field width specifier

I was asked recently whether the OpenCL printf supports the asterisk ("*") field width specifier. I believe the answer is "no", because the description in the OpenCL C spec only says:

> The field width takes the form of a nonnegative decimal integer...

whereas the C99 spec says:

> The field width takes the form of an asterisk * (described later) or a nonnegative decimal integer.

Note that the asterisk width specifier is also not in the list of tested printf format strings in teh CTS tests ([link](https://github.com/KhronosGroup/OpenCL-CTS/blob/master/test_conformance/printf/util_printf.cpp#L30)).

I think it would be clearer if this difference were included in the [Differences between OpenCL C and C99 printf](https://www.khronos.org/registry/OpenCL/specs/3.0-unified/html/OpenCL_C.html#differences-between-opencl-c-and-c99-printf) list.

While we're at it:

* We should also clarify that the asterisk precision specifier is not supported, similarly.
* We should describe the same restrictions in the SPIR-V environment specification.

Start with the linked OpenCL C “Differences between OpenCL C and C99 printf” section and the referenced test_conformance/printf/util_printf.cpp tests. Locate the corresponding SPIR-V environment specification text, then document the unsupported asterisk field-width and precision specifiers in both specifications and confirm the restrictions are consistent.

Written by the indexing model from the issue text.