awk: revert upstream's attempt to disallow hex strings

Upstream one-true-awk decided to disallow hex strings as numbers. This
is in line with awk's behavior prior to C99, and allowed by the POSIX
standard. The standard, however, allows them to be treated as numbers
because that's what the standard said in the 2001 through 2004 editions.
Since 2001, the nawk in FreeBSD has treated them as numbers, so restore
that behavior, allowed by the standard.

A number of scripts in the FreeBSD tree depend on this interpretation,
including scripts to build the kernel which had mysteriously started
failing for some people and not others. By re-allowing 0x hex numbers,
this fixes those scripts and restores POLA.

Upstream issue: https://github.com/onetrueawk/awk/issues/126
Sponsored by: Netflix
Reviewed by: kevans
MFC After: asap due to regression alrady merged to stable
Differential Revision: https://reviews.freebsd.org/D31199