ast_coredumper: allow setting asterisk binary explicitly

Adds an extra option, --asterisk-bin=<path> to ast_coredumper. If
provided, the binary given to gdb will be the parameter, rather than
asterisk from the PATH.

ASTERISK-27380 #close

Change-Id: I25f5b91eb75059b0fb2f142e468c26b283b0a9f3