toolchain: add user defined target LDFLAGS options

Allow the user to specify additional options for the target LDFLAGS.

I use this to pass the -te500v2 option to the linker, when using the
CodeSourcery toolchain for PowerPC. This chooses the correct CRT for e500 hard
float. Otherwise I get errors like

undefined reference to `_save32gpr_31'
undefined reference to `_rest32gpr_31_x'

at final link time.

[Peter: fixup, use qstrip]
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>