gdb.base/{prologue,symbol-alias}.exp C++ify

gdb.base/{prologue,symbol-alias}.exp C++ify

Make these testcases work when compiled as C++ programs.

These testcases use the alias attribute, which requires passing in the
target function's mangled name in C++.  To avoid having to figure out
how the functions are mangled, explicitly specify a linkage name.
This is preferred over 'extern "C"' because that doesn't work with
static functions.

gdb/testsuite/ChangeLog:

	* gdb.base/prologue.c [__cplusplus] (marker): Explicitly specify
	linkage name.
	* gdb.base/prologue.exp: Use print /d.
	* gdb.base/symbol-alias.exp: Handle C++ output.
	* gdb.base/symbol-alias2.c: Handle C++ output.
	[__cplusplus] (func): Explicitly specify linkage name.