Revert MIPS: Loongson: Fix build error when make modules_install

Revert commit 531b3d11 ("MIPS: Loongson: Fix build error when make
modules_install"), which made `-march=loongson2e', `-march=loongson2f',
and `-march=loongson3a' compilation options probed for even though GCC
has supported them since 4.4.0, 4.4.0, and 4.6.0 respectively, which is
below our current minimum requirement of 5.1, in an attempt to work
around for the `cc-option' `make' function being undefined with `make'
targets that do not use the compiler.  The workaround has now been made
obsolete, by querying the `need-compiler' variable instead so as to make
sure the compiler isn't called for non-build targets.

Verified with `fuloong2e_defconfig' and the `modules_install' target.

Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>