Fix missing files for ld when test suite not compiled in the source directory

Fix missing files for ld when test suite not compiled in the source directory

This patch fixes an issues with six test suite expect files that do not
run correctly when the test suite is not built in the source directory.  The
issue is these tests are not using the current "standard_testfile" call
but rather using the older set command to initialize the "testfile",
"srcfile" and "binprefix" variables or are missing the set for the
"binprefix" variable.

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gdb/testsuite/ChangeLog

2016-08-19  Carl Love  <cel@us.ibm.com>

	* gdb.arch/altivec-regs.exp: Use standard_testfile instead of
	maintaining separate logic for constructing the output path.
	* gdb.arch/powerpc-d128-regs.exp: Likewise.
	* gdb.arch/ppc-dfp.exp: Likewise.
	* gdb.arch/ppc-fp.exp: Likewise.
	* gdb.arch/vsx-regs.exp: Likewise.
	* gdb.arch/altivec-abi.exp: Likewise, plus added local variable
	binprefix for generating the additional binary files.