selftests/nolibc: add script to run testsuite

The script can run the testsuite for multiple architectures and provides
an overall test report.

Furthermore it can automatically download crosstools from
mirrors.kernel.org if requested by the user.

Example execution:

$ ./run-tests.sh
i386:          162 test(s): 162 passed,   0 skipped,   0 failed => status: success
x86_64:        162 test(s): 162 passed,   0 skipped,   0 failed => status: success
arm64:         162 test(s): 162 passed,   0 skipped,   0 failed => status: success
arm:           162 test(s): 162 passed,   0 skipped,   0 failed => status: success
mips:          162 test(s): 161 passed,   1 skipped,   0 failed => status: warning
ppc:           162 test(s): 162 passed,   0 skipped,   0 failed => status: success
ppc64:         162 test(s): 162 passed,   0 skipped,   0 failed => status: success
ppc64le:       162 test(s): 162 passed,   0 skipped,   0 failed => status: success
riscv:         162 test(s): 162 passed,   0 skipped,   0 failed => status: success
s390:          162 test(s): 161 passed,   1 skipped,   0 failed => status: warning
loongarch:     162 test(s): 161 passed,   1 skipped,   0 failed => status: warning

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20231105-nolibc-run-tests-v1-1-b59ff770a978@weissschuh.net