regex: Unnest nested functions in regcomp.c

This refactor moves four functions out of a nested scope and converts
them into static always_inline functions. collseqwc, table_size,
symb_table, extra are now initialized to zero because they are passed as
function arguments.

On x86-64, .text is 16 byte larger likely due to the 4 stores.
This is nothing compared to the amount of work that regcomp has to do
looking up the collation weights, or other functions.

If the non-buildable `sysdeps/generic/dl-machine.h` doesn't count,
this patch removes the last `auto inline` usage from glibc.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>