[Driver] Pass --target2= to linker from baremetal toolchain

According to the GNU ld manual
https://sourceware.org/binutils/docs/ld/ARM.html#ARM the R_ARM_TARGET2
relocation (used in exception handling tables) is treated differently
depending on the target. By default, LLD treats R_ARM_TARGET2 as
R_ARM_GOT_PREL (--target2=got-rel), which is correct for Linux but not
for embedded targets.

This patch adds --target2=rel to linker options in the baremetal
toolchain driver so that on baremetal targets, R_ARM_TARGET2 is
treated as R_ARM_REL32. Such behavior is compatible with GNU ld and
unwinding libraries (e.g., libuwind).

Reviewed By: peter.smith, phosek

Differential Revision: https://reviews.llvm.org/D149458