gdbsupport, gdb: add read_text_file_to_string, use it in linux_common_core_of_thread

gdbsupport, gdb: add read_text_file_to_string, use it in linux_common_core_of_thread

I would like to add more code to nat/linux-osdata.c that reads an entire
file from /proc or /sys and processes it as a string afterwards.  I
would like to avoid duplicating the somewhat error-prone code that reads
an entire file to a buffer.  I think we should have a utility function
that does that.

Add read_file_to_string to gdbsupport/filestuff.{c,h}, and make
linux_common_core_of_thread use it.  I want to make the new function
return an std::string, and because strtok doesn't play well with
std::string (it requires a `char *`, std::string::c_str returns a `const
char *`), change linux_common_core_of_thread to use std::string methods
instead.

Approved-By: Tom Tromey <tom@tromey.com>
Change-Id: I1793fda72a82969c28b944a84acb953f74c9230a