LoongArch: define preprocessing macros "__loongarch_{arch,tune}"

LoongArch: define preprocessing macros "__loongarch_{arch,tune}"

These are exported according to the LoongArch Toolchain Conventions[1]
as a replacement of the obsolete "_LOONGARCH_{ARCH,TUNE}" macros,
which are expanded to strings representing the actual architecture
and microarchitecture of the target.

[1] currently relased at https://github.com/loongson/LoongArch-Documentation
    /blob/main/docs/LoongArch-toolchain-conventions-EN.adoc

gcc/ChangeLog:

	* config/loongarch/loongarch-c.cc: Export macros
	"__loongarch_{arch,tune}" in the preprocessor.