gnatmake: Accept the `--sysroot=' GCC driver option

gnatmake: Accept the `--sysroot=' GCC driver option

According to `gnatmake' documentation:

"Any uppercase or multi-character switch that is not a 'gnatmake' switch
is passed to 'gcc' (e.g., '-O', '-gnato,' etc.)"

however the `--sysroot=' switch is actually rejected:

gnatmake: invalid switch: --sysroot=...

likely because it is one of the very few GCC driver options that have a
leading double dash and therefore we don't have a blanket fall-through
for such switches that would satisfy what our documentation claims.

The option is actually shared between the compiler and the linker, so
pass the switch to both build stages if requested, removing GNAT
testsuite issues like:

gnatmake: invalid switch: --sysroot=.../sysroot
compiler exited with status 1
Executing on host: .../gcc/gnatclean -c -q -n ./abstract_with_anonymous_result   (timeout = 300)
spawn -ignore SIGHUP .../gcc/gnatclean -c -q -n ./abstract_with_anonymous_result
PASS: gnat.dg/abstract_with_anonymous_result.adb (test for excess errors)
UNRESOLVED: gnat.dg/abstract_with_anonymous_result.adb compilation failed to produce executable

in a test environment where `--with-build-sysroot=.../sysroot' has been
used to build a cross-compiler.  Passing to the compilation stage only
would lead to errors like:

.../bin/riscv64-linux-gnu-ld: cannot find crt1.o: No such file or directory
.../bin/riscv64-linux-gnu-ld: cannot find -lc
collect2: error: ld returned 1 exit status
gnatlink: error when calling .../gcc/xgcc
gnatmake: *** link failed.
compiler exited with status 1
Executing on host: .../gcc/gnatclean -c -q -n ./abstract_with_anonymous_result   (timeout = 300)
spawn -ignore SIGHUP .../gcc/gnatclean -c -q -n ./abstract_with_anonymous_result
./abstract_with_anonymous_result.ali
./abstract_with_anonymous_result.o
FAIL: gnat.dg/abstract_with_anonymous_result.adb (test for excess errors)
Excess errors:
.../bin/riscv64-linux-gnu-ld: cannot find crt1.o: No such file or directory
.../bin/riscv64-linux-gnu-ld: cannot find -lc
gnatlink: error when calling .../gcc/xgcc

UNRESOLVED: gnat.dg/abstract_with_anonymous_result.adb compilation failed to produce executable

instead.

	gcc/ada/
	* make.adb (Scan_Make_Arg): Also accept `--sysroot=' for the
	compiler and the linker.

From-SVN: r275702