Fix mingw-w64 Ada native bootstrap (PR81878).

Fix mingw-w64 Ada native bootstrap (PR81878).

Due to the changes in PR81878 builds of GCC8 and trunk are impossible
with Ada enabled[1][2].

The reason the patch breaks the bootstrap is due to how gnatlink receives it's
arguments.

gnatlink is usually invoked as

        $(GNATLINK) -v gnatcmd -o ../../gnat$(exeext) \
          --GCC="$(CC) $(ADA_INCLUDES)" --LINK="$(GCC_LINK)" $(TOOLS_LIBS)

so it passes $(CC) and $(GCC_LINK) as quoted arguments to the program.
Because of this quotation the msys2 shell does not translate any paths in
$(CC) and $(GCC_LINK) from their Unix version to their Windows version.

Furthermore because there are multiple paths in the values separated by space
and because the paths often contain a prefix like -L (e.g. -L/f/foo) we can't
use `fix_srcfile_path` to fix this.

An alternative solution would have been to create a stub program that echos the
commandline options it receives back, and calling this program with $(CC) and $(GCC_LINK)
unquoted to get them translated.  However this was a bit more invasive.

So instead for native compilations we add -B../../ such that it picks up the lto plugin
from the previous built compiler.  Since it's native there shouldn't be a mismatch here.

[1] https://github.com/Alexpux/MINGW-packages/pull/3877#issuecomment-408651809
[2] https://gcc.gnu.org/ml/gcc/2018-07/msg00410.html

gnattools/ChangeLog:

	PR ada/81878
	* Makefile.in (TOOLS_FLAGS_TO_PASS_NATIVE): Add -B ../../.

From-SVN: r265583