Fix gdb.base/savedregs.exp with clang

Fix gdb.base/savedregs.exp with clang

gdb.base/savedregs.exp fails to run with clang, because of:

 gdb compile failed, /gdbtest/src/gdb/testsuite/gdb.base/savedregs.c:36:37:
       warning: operator '<<' has lower precedence than '+'; '+' will be
       evaluated first [-Wshift-op-parentheses]
   return callee (a1 << a2 * a3 / a4 + a6 & a6 % a7 - a8) + done;
                     ~~ ~~~~~~~~~~~~~^~~~
 /gdbtest/build/gdb/testsuite/gdb.base/savedregs.c:36:37: note: place
       parentheses around the '+' expression to silence this warning
   return callee (a1 << a2 * a3 / a4 + a6 & a6 % a7 - a8) + done;
                                     ^
                        (                )
 1 warning generated.

This commit fixes it by adding the suggested parentheses.

gdb/testsuite/ChangeLog:

	* gdb.base/savedregs.exp (caller): Use parentheses to
	make expected expression evaluation ordering explicit.