Backport a number of changes from lang/gcc5 and lang/gcc49:

• Move ONLY_FOR_ARCHS into the right position of the Makefile.
• Omit a comment describing GCC_VERSION and SUFFIX.
• Do not quote constant strings compared with ${ARCH}. [1]
• Last, but not least the only functional change (and an important one): Remove headers being created by GCC's fixincludes machinery from the installation / packaging to avoid breakage when FreeBSD's headers are changing afterwards. [2]

PR: 221905 [1], 222233 [2]
Submitted by: linimon [1]