fix: unblock --debug/--reldebug GPU builds (guard host s_mpi_abort out of device code)

s_compute_fast_magnetosonic_speed is a GPU routine (acc routine seq) but its
MFC_DEBUG diagnostic block called host s_mpi_abort, which is illegal in device
code without IPO inlining. Debug/reldebug builds disable IPO, so --gpu acc/mp
debug builds failed with NVFORTRAN-S-1061. Guard the abort with #ifndef MFC_GPU,
keeping the device-legal diagnostic print. CPU and Release builds are unchanged.

Author: sbryngelson

src/common/m_variables_conversion.fpp

@@ -1332,7 +1332,11 @@ contains
 #ifdef MFC_DEBUG
         if (disc < 0._wp) then
             print *, 'rho, c, Bx, By, Bz, h, term, disc:', rho, c, B(1), B(2), B(3), h, term, disc
+            ! s_mpi_abort is a host routine and cannot be called from device code
+            ! (this is a GPU routine); on GPU builds, emit the diagnostic print only.
+#ifndef MFC_GPU
             call s_mpi_abort('Error: negative discriminant in s_compute_fast_magnetosonic_speed')
+#endif
         end if
 #endif