sys: Disable C standards prior to C99 from kernel build

The kernel hasn't built with anything less than c99 for a long
time. Retire support in the build for it. In addition, retire the
translation of c99 to -std=iso9899:1999, since all latter day C
compilers that we support have had this for maybe 15 years or so (gcc
since 4.5, clang since the earliest version) and it simplifies the code.

Reviewed-by: imp, emaste
Differential-Revision: https://reviews.freebsd.org/D44145