gdb: rename start_symtab/end_symtab to start_compunit_symtab/end_compunit_symtab

gdb: rename start_symtab/end_symtab to start_compunit_symtab/end_compunit_symtab

It's a bit confusing because we have both "compunit_symtab" and "symtab"
types, and many methods and functions containing "start_symtab" or
"end_symtab", which actually deal with compunit_symtabs.  I believe this
comes from the time before compunit_symtab was introduced, where
symtab did the job of both.

Rename everything I found containing start_symtab or end_symtab to use
start_compunit_symtab or end_compunit_symtab.

Change-Id: If3849b156f6433640173085ad479b6a0b085ade2