PR10208 Support probing weak symbols

PR10208 Support probing weak symbols

*tapsets.cxx - Now always query the symtab (unless there is a pending interrupt
	       or dwarf callback error) on a function probe.  We need to be careful
	       to check probe point's we've already resolved which will already
	       have full debug information and to not place another probe there.
	       We've removed the case of probing the symbol table on a statement probe,
	       as that code was written specifically for the kernel without userspace
	       in mind and was resolving the function the statement resided in (causing
	       errors in some cases).

*list.exp    - Added testcase for weak symbols
*last_100_frees.stp   - we use @defined($mem) here because on 64 bit systems, the
		        wildcard search takes us through both 64 bit and 32 bit libc
		        (which doesn't have debuginfo), this means the probe point
		        resolved from the 32 bit library has no context info
*mutex-contention.stp - ditto but for @defined($mutex) and @defined($rwlock)