Synchronize GCC compile plugin headers

This patch copies some changes to the compile headers from GCC's
include/ directory.  It is the gdb equivalent of the GCC commit
bc0e18a9 -- however, while that commit also necessarily touched
libcc1, this one of course does not.

Tested by rebuilding and also running the gdb.compile tests.

Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31397