sparc: vdso: clean-up vdso Makefile

arch/sparc/vdso/Makefile is a replica of arch/x86/entry/vdso/Makefile.

Clean-up the Makefile in the same way as I did for x86:

 - Remove unnecessary export
 - Put the generated linker script to $(obj)/ instead of $(src)/
 - Simplify cmd_vdso2c

The corresponding x86 commits are:

 - 61615faf ("x86/build/vdso: Remove unnecessary export in Makefile")
 - 1742ed20 ("x86/build/vdso: Put generated linker scripts to $(obj)/")
 - c5fcdbf1

 ("x86/build/vdso: Simplify 'cmd_vdso2c'")

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: David S. Miller <davem@davemloft.net>