bpftrace: Fix __data_loc reads #122
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cherry-pick bpftrace/bpftrace@665b754
A __data_loc field is an (offset, len) pair; its value is determined by
adding the offset (in bytes) to the address of the tracepoint struct.
Commit bpftrace/bpftrace@06a8972 ("codegen: use preserve_static_offset intrinsic")
replaced the add instruction with getelementptr, but used i32 as the
GEP type without adjusting the offset. This causes the value to be
incorrectly set to ctx + 4*offset.
For example, this currently prints an empty or incomplete filename:
bpftrace -e 't:sched:sched_process_exec { print(str(args.filename)) }'
Fix it by changing the getelementptr type argument to i8.