gdb/testsuite: Fix many errors in gdb.reverse with clang

Clang does not add line information for lines that only contain a
closing } in functions. Many tests in the gdb.reverse folder set a
breakpoint in that line, but don't seem to use information available
after the return statement is executed, so this commit moves the
breakpoint to the previous line, where the return statement is.

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