xbps-src common.sh: add FCC for gfortran
xbps-src common.sh: add FCC for gfortran

Introduce an environment variable `FCC` for fortran, just as CC,
CXX etc. are defined for the other compilers.

It is set to `${XBPS_CROSS_TRIPLET}-gfortran` when cross compiling, or
to just `gfortran` when building for the native architecture.
Use just "$FCC" now when specifying the fortran compiler in a template.

coredumb/void-packages,liwakura/void-packages,kulinacs/void-packages,kulinacs/void-packages,gangsterakato/void-packages,jprjr/void-packages,WKuipers/void-packages,ayghor/void-packages,NeoChontrix/void-packages,jprjr/void-packages,coredumb/void-packages,Bubble-be/void-packages,Bubble-be/void-packages,liwakura/void-packages,ayghor/void-packages,opsaaspo/void-packages,necrophcodr/void-packages,EaterOfCode/void-packages,kulinacs/void-packages,juliendehos/void-packages,floGik/void-packages,gangsterakato/void-packages,ayghor/void-packages,kulinacs/void-packages,liwakura/void-packages,juliendehos/void-packages,jprjr/void-packages,necrophcodr/void-packages,liwakura/void-packages,floGik/void-packages,coredumb/void-packages,NeoChontrix/void-packages,floGik/void-packages,necrophcodr/void-packages,kulinacs/void-packages,floGik/void-packages,jprjr/void-packages,WKuipers/void-packages,coredumb/void-packages,EaterOfCode/void-packages,Bubble-be/void-packages,ayghor/void-packages,EaterOfCode/void-packages,Bubble-be/void-packages,opsaaspo/void-packages,jprjr/void-packages,coredumb/void-packages,opsaaspo/void-packages,EaterOfCode/void-packages,ayghor/void-packages,WKuipers/void-packages,gangsterakato/void-packages,necrophcodr/void-packages,liwakura/void-packages,opsaaspo/void-packages,gangsterakato/void-packages,floGik/void-packages,liwakura/void-packages,jprjr/void-packages,NeoChontrix/void-packages,floGik/void-packages,Bubble-be/void-packages,NeoChontrix/void-packages,Bubble-be/void-packages,NeoChontrix/void-packages,EaterOfCode/void-packages,WKuipers/void-packages,coredumb/void-packages,WKuipers/void-packages,juliendehos/void-packages,juliendehos/void-packages,ayghor/void-packages,juliendehos/void-packages,WKuipers/void-packages,gangsterakato/void-packages,opsaaspo/void-packages,NeoChontrix/void-packages,necrophcodr/void-packages,juliendehos/void-packages,kulinacs/void-packages,gangsterakato/void-packages,opsaaspo/void-packages