[MSP430] Use R_MSP430_16_BYTE type for FK_Data_2 fixup

[MSP430] Use R_MSP430_16_BYTE type for FK_Data_2 fixup

Linker fails to link example like this (simplified case from newlib
sources):

$ cat test.c

extern const char _ctype_b[];
struct _t { char *ptr; };
struct _t T = { ((char *) _ctype_b + 3) };
$ cat ctype.c

char _ctype_b[4] = { 0, 0, 0, 0 };
LD: test.o:(.data+0x0): warning: internal error: unsupported relocation error

We also follow gnu toolchain here, where 2-byte relocation mapped to
R_MSP430_16_BYTE, instead of R_MSP430_16.

Patch by Kristina Bessonova!

Differential Revision: https://reviews.llvm.org/D54620

llvm-svn: 347074