Reimplement tui_get_begin_asm_address

tui_get_begin_asm_address looks for the inferior's "main" to display
it.  I think this is incorrect in two ways.

First, it should probably instead use the user's most recent source
context, if one has been set.

Second, it uses a hard-coded list of "main" names, but gdb already has
a better approach to handling this.

This patch fixes both of these problems.

gdb/ChangeLog
2019-12-20  Tom Tromey  <tom@tromey.com>

	* tui/tui-disasm.c (tui_get_begin_asm_address): Use
	get_current_source_symtab_and_line, and main_name.

Change-Id: I77dc13d49148e8dec5aa3eeb357ce3968a68d0bd