[gdb] Fix regoff_t incompatibility

I did an experiment with importing the regex module in gnulib, and trying to
build gdb.

The first problem I ran into was that:
- regoff_t was defined as long int, and
- the address of a regoff_t variable i in ui_file_style::parse was passed
  as int * to function extended_color.

Fix this by changing the types of some function parameters of functions
read_semi_number and extended_color from int * to regoff_t *.

Tested on x86_64-linux.

gdb/ChangeLog:

2021-04-13  Tom de Vries  <tdevries@suse.de>

	* ui-style.c (read_semi_number, extended_color): Change idx parameter
	type to regoff_t *.