Add loongarch64 support

[Dandan336]

Hi maintainers,

Compiling the lnav failed for loong64 in the Debian Package Auto-Building environment.
The error log is as follows,

In file included from yaml-extension-functions.cc:36:
./third-party/rapidyaml/ryml_all.hpp:514:5: error: #error "unknown CPU architecture"
  514 |    #error "unknown CPU architecture"
      |     ^~~~~
g++ -std=c++17 -DHAVE_CONFIG_H -I. ......

The full build log can be found at https://buildd.debian.org/status/logs.php?pkg=lnav&ver=0.12.3-1&arch=loong64.

Please review this commit.
Based on this commit, lnav 0.12.3-1 source package was built successfully on locally.

Testsuite summary for lnav 0.12.3
=====================================
# TOTAL: 40
# PASS:  40
# SKIP:  0
# XFAIL: 0
# FAIL:  0
# XPASS: 0
# ERROR: 0
=====================================
......
   dh_builddeb
dpkg-deb: building package 'lnav-dbgsym' in '../lnav-dbgsym_0.12.3-1_loong64.deb'.
dpkg-deb: building package 'lnav' in '../lnav_0.12.3-1_loong64.deb'.
 dpkg-genbuildinfo -O../lnav_0.12.3-1_loong64.buildinfo
 dpkg-genchanges -O../lnav_0.12.3-1_loong64.changes
......

Your opinions are welcome.

Best regards,
Dandan Zhang

[tstack]

Thanks!