builtin: make inf/nan conditional depending on float support

[mauke]

On some machines (VAX), the double type doesn't support Infinity/NaN values. Handle this case by making builtin::inf/builtin::nan throw a runtime error.

Fixes #22882.

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• This set of changes requires a perldelta entry, and it is included.

[bulk88]

This post isn't saying anything is wrong or needs changes, in this ticket/commit.

How does Perl handle pack() and wire/disk data for interchange on VAX? Why not add the ieee754 bit patterns as some kind of specific error instead of a more confusing feature not found error?

Google says VAX does have the NaN feature in its CPU, some quotes

An exponent value of zero, together with a sign bit of 1, is taken as reserved. Floating-point instructions processing a reserved operand take a reserved operand fault (see [Appendix E]).

This is a reserved operand. It causes a reserved operand fault if used in a subsequent floating-point instruction. The N and V condition code bits in the PSL are set, and the Z and C bits in the PSL are cleared. The type code pushed onto the stack is 3 (SS$_FLTOVF).

http://odl.sysworks.biz/disk$axpdocdec011/opsys/vmsos73/vmsos73/4515/4515pro_013.html