gccrs: Update llvm_builtins test cases

gcc/testsuite/ChangeLog:

	* rust/compile/llvm_builtins.rs: Removed 64-bit functions, ensure main calls all llvm
	built-ins, add dg-final directives to check the generated tree dump, ensure the test
	only gets compiled on x86 targets.
	* rust/compile/llvm_builtins_64_bit.rs: New test derived from llvm_builtins.rs, but
	for 64-bit functions.

Signed-off-by: Yap Zhi Heng <yapzhhg@gmail.com>