gdb/testsuite: Fix style.exp failures on targets without argc/argv support

Some embedded targets don't have full support for argc/argv.  argv
may print as "0x0" or as an address with a symbol name following.
This causes problems for the regexps in the style.exp line-wrapping
tests that assume it always prints as an ordinary address in backtrace
output.

This patch generalizes the regexps to handle these additional forms
and reworks some of the line-wrapping tests to account for the argv
address string being shorter or longer than a regular address.

Reviewed-By: Tom Tromey <tom@tromey.com>
Approved-By: Tom Tromey <tom@tromey.com>