base/binutils: Cleanup packing lists.

• Use BUREMOVE to strip binutils tools not installed by the base/binutils package.
• Update BUREMOVE logic in devel/binutils to cope with the base package which installs tools without a BUTARGET- prefix.
• Use MANPREFIX for BUREMOVE to handle the PREFIX=/usr case used by base/binutils.
• Remove binutils headers and libraries explicitly from the staging area for base/binutils.
• Add missing plist entries for binutils binaries installed under a BUTARGET subdirectory.

Approved by: bapt (implicit for base/*)
Differential Revision: https://reviews.freebsd.org/D16464