Define L_ENDIAN for linux64-loongarch64

Define L_ENDIAN for linux64-loongarch64

In commit d7c0fc5b1a7b5cb2219f8d89a861f3879582fc16 we removed L_ENDIAN
definition for guessed linux64-loongarch64 as it had caused an
inconsistency between configurations with and without explicit
specifying linux64-loongarch64.  Now add it back to the proper location.

Unlike MIPS or RISC-V, LoongArch is always little-endian [1].

By the way, change "LOONGARCH" to "LoongArch" in a comment as LOONGARCH
should only appear in the identifiers of macros, constants, etc.

[1]:https://loongson.github.io/LoongArch-Documentation/LoongArch-Vol1-EN.html#endian

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/23064)