regexec: remove alloca usage in build_trtable

It syncs with gnulib version 1731fef3d.  On build_trtable prevent
inlining, so that it doesn't bloat the caller's stack and use auto
variables instead of alloca/malloc.

After these changes, build_trtable's total stack allocation is
only 20 KiB on a 64-bit machine, and this is less than glibc's 64
KiB cutoff so there's little point to using alloca to shrink it.

Checked on x86_64-linux-gnu.