microblaze: Resolve non-relocatable branch in pt-vfork.S (BZ#21779)

The relative branch directly to __libc_vfork results in an relocation
that cannot be resolved. Specifically a R_MICROBLAZE_64_PCREL relocation
is created for this branch, however for MicroBlaze R_MICROBLAZE_64_PCREL
type relocations symbols are not resolved. Additionally due to the
branch being located in the .text section the instruction cannot be
rewritten as the section is not writable, and causes a segfault at
runtime when loading libpthread.

To resolve this issue, ensure the branch is done using PLT. This removes
the need to modify the instruction and trades the R_MICROBLAZE_64_PCREL
for a more common R_MICROBLAZE_JUMP via the PLT.

	[BZ #21779]
	* sysdeps/unix/sysv/linux/microblaze/pt-vfork.S: Branch using PLT.