lang/ghc: Update arm bootstrap

The ghc bootstrap (v8.6.5) on arm requires devel/llvm60,
this llvm version is obsolete and still uses python2.7 to build,
update the bootstrap to a newer version (v8.10.4) that requires devel/llvm90

Bump bootstrap to 8.10.4 on arm
Bump minimum required FreeBSD version, 12.1 is not supported anymore
Drop unecessary RUN_DEPENDS
Collapse arm ARCH conditionnal

Approved by: arrowd
Differential Revision: https://reviews.freebsd.org/D28906