Fix testcases with required but unreferenced functions and variables

A number of testcases define variables and/or functions which are
referenced by GDB during the test, but which are not referenced from
within the test executable.  Clang correctly recognizes that these
variables and functions are unused, and optimizes them out, causing
the testcases in question to fail.  This commit adds __attribute__
((used)) in various places to prevent this.

gdb/testsuite/ChangeLog:

	* gdb.base/msym-bp.c (foo): Add __attribute__ ((used)).
	* gdb.base/msym-bp-2.c (foo): Likewise.
	* gdb.base/msym-lang.c (foo): Likewise.
	* gdb.base/msym-lang-main.c (foo): Likewise.
	* gdb.base/symtab-search-order-1.c (static_global): Likewise.
	* gdb.guile/scm-pretty-print.c (eval_func): Likewise.
	* gdb.mi/mi-sym-info-1.c (global_f1): Likewise.
	* gdb.mi/mi-sym-info-2.c (global_f1, var1, var2): Likewise.
	* gdb.multi/watchpoint-multi-exit.c (globalvar): Likewise.
	* gdb.python/py-as-string.c (enum_valid, enum_invalid): Likewise.
	* gdb.python/py-objfile.c (static_var): Likewise.
	* gdb.python/py-symbol.c (rr): Likewise.
	* gdb.python/py-symbol-2.c (anon, rr): Likewise.
	* gdb.mi/mi-sym-info.exp (lineno1, lineno2): Updated.