Makefile.arm: remove -march flags

Makefile.arm: remove -march flags

The provided -march flags, especially for ARMv5 and ARMv6 may not
necessarily match the needed ones: for ARMv5, it might be armv5,
armv5te, armv5t, etc. If the wrong one is used, the incorrect toolchain
sysroot can be used in a multilib toolchain.

Therefore, let the user building OpenBLAS pass the appropriate -march
flag.

The other flags, such as -mfpu=vfp or -mfloat-abi=hard are kept, as they
are actually required for the build to proceed (OpenBLAS uses VFP
instructions, and assume an EABIhf ABI).

[Peter: update for v0.2.20]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>
[Retrieved from:
https://git.buildroot.net/buildroot/tree/package/openblas/0001-Makefile.arm-remove-march-flags.patch]
Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>