perf annotate-data: Collect global variables without name

perf annotate-data: Collect global variables without name

Previously, global_var__collect() required get_global_var_info() to
succeed (i.e., the variable must have a symbol name) before caching a
global variable. This prevented variables that exist in DWARF but lack
symbol table coverage from being cached.

Remove the symbol table requirement since DW_OP_addr already provides
the variable's address directly from DWARF. The symbol table lookup is
now optional to obtain the variable name when available.

Also remove the var_offset != 0 check, which was intended to skip
variables where the access address doesn't match the symbol start. The
symbol table lookup is now optional and I found removing this check has
no effect on the annotation results for both kernel and userspace
programs.

Test results show improved annotation coverage especially for userspace
programs with RIP-relative addressing instructions.

Signed-off-by: Zecheng Li <zli94@ncsu.edu>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>