Fix gdb.base/print-file-var.exp with Clang

Fix gdb.base/print-file-var.exp with Clang

The C++ parts of gdb.base/print-file-var.exp failed to build with
Clang because the "-x c++" option added by gdb_compile caused the
compiler to attempt to parse .so files as C++.  This commit splits
the compiler and linker options into separate lists, and switches
to building via build_executable_from_specs which can accommodate
this separation.

gdb/testsuite/ChangeLog:

	* gdb.base/print-file-var.exp (test): Separate compiler and
	linker options, and build using build_executable_from_specs
	to accommodate this.