syscalls: Correct the type of _k_syscall_table

_k_syscall_table is an array of function pointers and is declared as
such in C sources, this makes it an STT_OBJECT[0] in the symbol
table. But when the same symbol is declared in assembly, it is
declared to be a function, which would make the symbol an STT_FUNC.

When linking with LTO this type inconsistency results in the warning:

real-ld: Warning: type of symbol `_k_syscall_table' changed from 2 to
1 in /tmp/cc84ofK0.ltrans8.ltrans.o

To fix this warning we declare the table with GDATA instead of GTEXT,
which will change the type from 'function' to 'object'.

[0]
https://docs.oracle.com/cd/E19455-01/816-0559/chapter6-79797/index.html



Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>