RISC-V: testsuite: ensure vtype is call clobbered

RISC-V: testsuite: ensure vtype is call clobbered

Per classic Vector calling convention ABI, vtype is call clobbered,
so ensure gcc regenerates a VSETVLI in following cases:
 - after a function call.
 - after an inline asm fragment which clobbers vtype.

ATM gcc seems to be doing the right thing, but a test can never hurt.

gcc/testsuite/ChangeLog:
	* gcc.target/riscv/rvv/vtype-call-clobbered.c: New Test.

Signed-off-by: Vineet Gupta <vineetg@rivosinc.com>