[HACK] Revert "riscv: Enable BUILDTIME_TABLE_SORT"

This reverts commit 54fed35f to resolve
the following build error on Ubuntu 16.04 host:

scripts/sorttable.c: In function ‘do_file’:
scripts/sorttable.c:352:7: error: ‘EM_RISCV’ undeclared (first use in this function)
  case EM_RISCV:
       ^
scripts/sorttable.c:352:7: note: each undeclared identifier is reported only once for each function it appears in

scripts/Makefile.host:95: recipe for target 'scripts/sorttable' failed
make[2]: *** [scripts/sorttable] Error 1
make[2]: *** Waiting for unfinished jobs....

Our CI runs on Ubuntu 16.04 and this revert helps the nightly CI to
progress.