c: Add support for __FILE_NAME__ macro (PR c/42579)

The toolchain provided by ST for stm32 has had support for
__FILENAME__ for a while, but clang/llvm has recently implemented
support for __FILE_NAME__, so it seems better to use the same macro
name in GCC.

It happens that the ST patch is similar to the one proposed in PR
c/42579.

Given these input files:
::::::::::::::
mydir/myinc.h
::::::::::::::
char* mystringh_file = __FILE__;
char* mystringh_filename = __FILE_NAME__;
char* mystringh_base_file = __BASE_FILE__;
::::::::::::::
mydir/mysrc.c
::::::::::::::

char* mystring_file = __FILE__;
char* mystring_filename = __FILE_NAME__;
char* mystring_base_file = __BASE_FILE__;

we produce:
$ gcc mydir/mysrc.c -I . -E
char* mystringh_file = "./mydir/myinc.h";
char* mystringh_filename = "myinc.h";
char* mystringh_base_file = "mydir/mysrc.c";

char* mystring_file = "mydir/mysrc.c";
char* mystring_filename = "mysrc.c";
char* mystring_base_file = "mydir/mysrc.c";

2021-05-20  Christophe Lyon  <christophe.lyon@linaro.org>
	    Torbjörn Svensson  <torbjorn.svensson@st.com>

	PR c/42579
	libcpp/
	* include/cpplib.h (cpp_builtin_type): Add BT_FILE_NAME entry.
	* init.c (builtin_array): Likewise.
	* macro.c (_cpp_builtin_macro_text): Add support for BT_FILE_NAME.

	gcc/
	* doc/cpp.texi (Common Predefined Macros): Document __FILE_NAME__.

	gcc/testsuite/
	* c-c++-common/spellcheck-reserved.c: Add tests for __FILE_NAME__.
	* c-c++-common/cpp/file-name-1.c: New test.