Support golang arguments

[brendangregg]

using gccgo, Go function arguments can be read using arg0...argN, as it follows the standard AMD64 ABI. Eg:

# bpftrace -e 'uprobe:/home/bgregg/func:main.add { printf("%d %d\n", arg0, arg1); }'
Attaching 1 probe...
42 13

But using Go gc, it follows the Plan9 stack convention where everything is placed on the stack. The same arguments can be read instead using:

# bpftrace -e 'uprobe:/home/bgregg/Lang/go/func:main*add { printf("%d %d\n", *(reg("sp") + 8), *(reg("sp") + 16)); }'
Attaching 1 probe...
42 13

This ticket is to suggest adding these aliases:

• goarg0 == *(reg("sp") + 8)
• goarg1 == *(reg("sp") + 16)
• etc

Maybe better: call them sarg0, sarg1, ...: short for "stack argument", since Golang may not be the only runtime/compiler doing this.