MFH: r565472

sysutils/memtest86+: remove dependency on gcc48

This is one of three ports still explicitly dependent on lang/gcc48. The
problems that were preventing the upgrade were as follows:

• The OUT*/IN* macros in io.h were declaring the produced functions as extern, yielding multiple "definitions." They're inline asm, so just give them static linkage.

• reboot was declared inline with non-static linkage, thus leaving it in a weird state where it wasn't visible within the same CU or another CU. Drop the inline for now; if it really needs to be inlined, it can be moved into a header and declared static inline.

I made these changes, then discovered there's a 5.31b that's still in
testing. I checked the archive for that and was delighted to discover these
changes had already actually been made there, too, so let's consider this a
backport.

PR: 253303
Approved by: avg (maintainer)