gdb: use std::string instead of a fixed size buffer

The 'section' command uses a fixed size buffer into which a section
name is copied.  This commit replaces this with a use of std::string
so we can now display very long section names.

The expected results of one test need to be updated.

gdb/ChangeLog:

	* exec.c (set_section_command): Move variable declarations into
	the function body, and use std::string instead of a fixed size
	buffer.

gdb/testsuite/ChangeLog:

	* gdb.base/sect-cmd.exp: Update expected results.