gccrs: Emit an error message on unsupported llvm_asm

gccrs: Emit an error message on unsupported llvm_asm

llvm_asm was never meant to be completely supported since it has been
replaced with the asm macro but we still need it to compile some parts
of libcore, previously the compiler was aborting when an unsupported
llvm_asm construct was found.

gcc/rust/ChangeLog:

	* ast/rust-expr.h: Add const getters to llvm members.
	* hir/rust-ast-lower-expr.cc (check_llvm_asm_support): Check llvm_asm
	usage validity.
	(ASTLoweringExpr::visit): Emit an error message instead of aborting.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>