sigsegv.c: enable build for LoongArch

[212dandan]

When compiling the package a2jmidid for loong64 in the Debian Package Auto-Building environment 1, The error message is as follows:
../sigsegv.c: In function ‘signal_segv’:
../sigsegv.c:97:20: error: ‘NGREG’ undeclared (first use in this function)
97 | for(i = 0; i < NGREG; i++)
| ^~~~~
../sigsegv.c:97:20: note: each undeclared identifier is reported only once for each function it appears in
In file included from ../sigsegv.c:39:
../sigsegv.c:106:39: error: ‘mcontext_t’ has no member named ‘gregs’; did you mean ‘__gregs’?
106 | ucontext->uc_mcontext.gregs[i]
| ^~~~~
../log.h:47:134: note: in definition of macro ‘a2j_error’

Please review.
If you have any questions, you can contact me at any time.