loongarch: testsuite: adapt stack-usage-1.c for LP64

loongarch: testsuite: adapt stack-usage-1.c for LP64

LoongArch backend allocates two additional 8-byte stack slots for LP64,
one for saving $fp and another for saving the temporary value "1".
Ideally they are both unneeded, but (1) we're using -O0 so the code is
suboptimized by the nature; (2) any improvement (if possible) should be
deferred to GCC 13.  So for now simply adjust the test to make it pass.

gcc/testsuite/

	* gcc.dg/stack-usage-1.c: Adjust for LoongArch LP64.