gdb/testsuite: account for clang's nested destructor calls on gdb.cp/mb-ctor.exp

gdb/testsuite: account for clang's nested destructor calls on gdb.cp/mb-ctor.exp

When compiling virtual classes's destructors, two versions are compiled,
one with a single parameter (this) and the other with 2 parameters (this
and vtt).

GCC's compilation makes it so either the version with 1
parameter or the one with 2 parameters is called, depending on whether
the destructor is being called by the class itself or by an inherited
class. On the test gdb.cp/mb-ctor.exp, this means that the breakpoint
set at the destructor will be hit 4 times.

Clang, on the other hand, makes the single-parameter version call the 2
parameter version, probably in an attempt to reduce the size of the
resulting executable. This means that the gdb.cp/mb-ctor.exp will hit 6
breakpoints before finishing, and is the reason why this test was
failing. To make this test stop failing, a compiler check is added and
another "continue" instruction is issued to account for this difference.

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