loongarch64: specify -mno-relax when supported
The LoongArch64 TSD assembly code relies on accurate .balign, which will be broken by linker relaxation.
Specify -mno-relax when this option is supported. When this option is not supported, assume linker relaxation is not available and nothing is needed.
A new autoconf macro is introduced from autoconf-archive for easily handling testing compiler options.
Signed-off-by: Icenowy Zheng uwu@icenowy.me