gccrs: Set the default ABI to C for extern blocks and extern functions

gccrs: Set the default ABI to C for extern blocks and extern functions

Previously, the default ABI was set to Rust, which is not correct for
extern blocks and extern functions. This patch changes the default
ABI to C for these cases.

gcc/rust/ChangeLog:

	* hir/rust-ast-lower-base.cc (ASTLoweringBase::lower_qualifiers):
	Change default ABI to C for extern functions
	(ASTLoweringBase::lower_extern_block): Likewise

Signed-off-by: Nobel Singh <nobel2073@gmail.com>