selftests/nolibc: always build sparc32 tests with -mcpu=v8

selftests/nolibc: always build sparc32 tests with -mcpu=v8

Since LLVM commit 39e30508a7f6 ("[Driver][Sparc] Default to -mcpu=v9 for
32-bit Linux/sparc64 (#109278)"), clang defaults to -mcpu=v9 for 32-bit
SPARC builds. -mcpu=v9 generates instructions which are not recognized
by qemu-sparc and qemu-system-sparc.

Explicitly enforce -mcpu=v8 to generate compatible code.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://patch.msgid.link/20260106-nolibc-sparc32-fix-v2-1-7c5cd6b175c2@weissschuh.net