rs6000: avoid peeking eof after __vector

rs6000: avoid peeking eof after __vector

There is a rare corner case: where vector is followed only by one
valid identifer and the ";" which is near the end of the file.

Like the case in PR101168:
using vdbl =  __vector double;
#define BREAK 1

For this case, "vector double" is followed by CPP_SEMICOLON and then
EOF.  There is no more tokens need to check for this case.

	PR preprocessor/101168

gcc/ChangeLog:

	* config/rs6000/rs6000-c.cc (rs6000_macro_to_expand):
	Avoid empty identifier.

gcc/testsuite/ChangeLog:

	* g++.target/powerpc/pr101168.C: New test.