package/gcc/9.3.0: fix host-gcc-final when ccache is used

package/gcc/9.3.0: fix host-gcc-final when ccache is used

As reported by several Buildroot users [1][2][3], the gcc build
may fail while running selftests makefile target.

The problem only occurs when ccache is used with gcc 9 and 10,
probably due to a race condition.

While debuging with "make -p" we can notice that s-selftest-c target
contain only "cc1" as dependency instead of cc1 and SELFTEST_DEPS [4].

  s-selftest-c: cc1

While the build is failing, the s-selftest-c dependencies recipe is
still running and reported as a bug by make.

  "Dependencies recipe running (THIS IS A BUG)."

A change [5] in gcc 9 seems to introduce the problem since we can't
reproduce this problem with gcc 8.

As suggested by Yann E. MORIN [6], move SELFTEST_DEPS before
including language makefile fragments.

With the fix applied, the s-seltest-c dependency contains
SELFTEST_DEPS value.

  s-selftest-c: cc1 xgcc specs stmp-int-hdrs ../../gcc/testsuite/selftests

[1] http://lists.busybox.net/pipermail/buildroot/2020-May/282171.html
[2] http://lists.busybox.net/pipermail/buildroot/2020-May/282766.html
[3] https://github.com/cirosantilli/linux-kernel-module-cheat/issues/108
[4] https://gcc.gnu.org/git/?p=gcc.git;a=blob;f=gcc/c/Make-lang.in;h=bfae6fd2549c4f728816cd355fa9739dcc08fcde;hb=033eb5671769a4c681a44aad08a454e667e08502#l120
[5] https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=033eb5671769a4c681a44aad08a454e667e08502
[6] http://lists.busybox.net/pipermail/buildroot/2020-May/283213.html

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Ben Dakin-Norris <ben.dakin-norris@navtechradar.com>
Cc: Maxim Kochetkov <fido_max@inbox.ru>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>