[Support for trace debugging: registers that were not collected.]
* remote.c (remote_fetch_registers): accept 'xxxx' in the register
packet, with the meaning "register value is not available".
Set register_valid to -1, which will connote "no value available".
* findvar.c (read_relative_register_raw_bytes): return failure if
register_valid == -1. (value_of_register): return failure if
register_valid == -1. (read_var_value): return error if
value_of_register fails for a register variable.
(value_from_register): return failure if register_valid == -1.
* eval.c (evaluate_subexp_standard): return error if
value_of_register fails for a register used in an expression.
* infcmd.c (do_registers_info): display "value not available"
for registers for which register_valid == -1.
* tracepoint.c (set_raw_tracepoint): just save the filename as is
from the symbol table, rather than trying to prepend the dir name.
Also save the bfd section. (tracepoints_info): use the section
when looking up the function name.
* tracepoint.h: add section field to tracepoint struct.
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