Accept gdb.Value in more Python APIs

PR python/27000 points out that gdb.block_for_pc will accept a Python
integer, but not a gdb.Value.  This patch corrects this oversight.

I looked at all uses of GDB_PY_LLU_ARG and fixed these up to use
get_addr_from_python instead.  I also looked at uses of GDB_PY_LL_ARG,
but those seemed relatively unlikely to be useful with a gdb.Value, so
I didn't change them.  My thinking here is that a Value will typically
come from inferior memory, and something like a line number is not too
likely to be found this way.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=27000