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.