draft for loongarch64

[xyq1113723547]

autobuild3: initial adaptation for loongarch64 port; #119

• tentative loongarch64 GCC compile flags and toolchain placeholder
• add arch/loongarch64.sh
• update arch/_common.sh to invoke toolchain placeholder
• delete meaningless mfix-loongson3-llsc flag for loongarch

[MingcongBai]

@xyq1113723547 Please reformat your commits in accordance to our Styling Manual.

[xyq1113723547]

@MingcongBai changed commits msgs

[MingcongBai]

Thanks!