gccrs: Fix legacy mangling to use Unicode escape

[tamaroning]

Fix #2545

Changlog

gccrs: Fix legacy mangling to use Unicode escape
gcc/rust/ChangeLog:

	* backend/rust-mangle.cc (legacy_mangle_name): Use Unicode escape

Result of mangling:

input:

pub fn あ() {}
mod Module {
    pub fn function_日本語_русский() {}
}
fn main() {
    あ();
    Module::function_日本語_русский();
}

output:

...
_ZN1a7$u3042$17hae3ccdfa81d818e4E:
	.cfi_startproc
...
_ZN1a6Module79function_$u65e5$$u672c$$u8a9e$_$u440$$u443$$u441$$u441$$u43a$$u438$$u438$$u439$17hae3ccdfa81d818e4E:
	.cfi_startproc
...
main:
.LFB2:
	.cfi_startproc
...

Demangle result:

_ZN1a6Module79function_$u65e5$$u672c$$u8a9e$_$u440$$u443$$u441$$u441$$u43a$$u438$$u438$$u439$17hae3ccdfa81d818e4E
=> a::Module::function_日本語_русскиий::hae3ccdfa81d818e4
_ZN1a7$u3042$17hae3ccdfa81d818e4E:
=> a::あ::hae3ccdfa81d818e4

Checked with https://online-demangler-iv5u2bxxb-tamaroning.vercel.app/

[philberty]

LGTM