risc-v: Move common memory map to its own file from riscv_internal

[pussuw]

Move the linker defined symbols to a separate file, so they can be
accessed without pulling in everything from riscv_internal.h and
whatever it includes (e.g. syscall.h drags in a lot).

Summary

Split the linker defined symbols from riscv_internal.h. Main reason is to allow the xxx_memorymap.h files to include this instead of pulling in the entire riscv_internal.h file (and what it includes)

Impact

Minor refactoring

Testing

icicle

[pussuw]

Don't think the error is my doing

curl: (28) Failed to connect to www.phyplusinc.com port 80: Connection timed out
cp: cannot stat 'libphy62xxble.a': No such file or directory
make[1]: *** [chip/Make.defs:82: .buildlib] Error 1
make[1]: Target 'context' not remade because of errors.
make: *** [tools/Unix.mk:425: arch/arm/src/.context] Error 2
make: Target 'all' not remade because of errors.

[acassis]

Don't think the error is my doing

curl: (28) Failed to connect to www.phyplusinc.com port 80: Connection timed out
cp: cannot stat 'libphy62xxble.a': No such file or directory
make[1]: *** [chip/Make.defs:82: .buildlib] Error 1
make[1]: Target 'context' not remade because of errors.
make: *** [tools/Unix.mk:425: arch/arm/src/.context] Error 2
make: Target 'all' not remade because of errors.

It is not! Maybe we need to have some mirror to avoid these sites disappear and we miss the closed source/proprietary libraries...

[pussuw]

The fix for the build error is here:
#6530