OpenACC 2.7: update _OPENACC symbol to 201811

OpenACC 2.7: update _OPENACC symbol to 201811

This patch updates the _OPENACC preprocessor symbol to "201811",
to indicate OpenACC 2.7 support.

2025-04-11  Chung-Lin Tang  <cltang@baylibre.com>

gcc/c-family/ChangeLog:

	* c-cppbuiltin.cc (c_cpp_builtins): Updated _OPENACC to "201811"
	for OpenACC 2.7.

gcc/fortran/ChangeLog:

	* cpp.cc (cpp_define_builtins): Updated _OPENACC to "201811"
	for OpenACC 2.7.